TRANSMISSION 
'OF  RADIANT  ENERGY 

BY 

OPHTHALMIC   GLASSES 


Being  an  Essay  Contributed 
to 

The  American  Encyclopedia  of  Ophthalmology 

by 
CHARLES  SHEARD,  A.B.,  A.M.,  Ph.D. 

Physiological  Opticist.  The  American  Optical  Company;  formerly  Professor  and 
Director  and  now  Non-resident  Professor  of  Applied  Optics,  The  Ohio  State 
University;  Collaborator  in  The  American  Encyclopedia  of  Ophthalmology; 
Member  of  the  Optical  Society  of  America,  The  Physical  Society, 
Honorary  Member  of  the  American  Optometric  Association,  etc.; 
Author  of  Physiological  Optics  (1918),  Dynamic  Ocular  Tests 
(1917),  Dynamic  Skiametry  (1920),  Ocular  Accommodation 
(1920)  and  series  of  articles  on  Mathematical  Studies  in 
Optics  (1910-11),  Cylindrical  Lenses  (1914),  and 
several  contributions  on  The  lonization  from 
Hot  Salts  and  Metals,  The  Free  Vibra- 
tions of  Lecher  Systems,  etc.,  Edi- 
tor of  The  American  Journal  of 
Physiological  Optics 


Illustrated  with  Seventy-nine  Diagrams 


Chicago 

CLEVELAND  PRESS 
1921 


.  .  -    -  .  .  •   -  r   i 

•  .•  .....  ,   . 

•  •    •  »/  »  3    • 
»  *»«         .    .     ..-,•., 


DEDICATED   TO 

ALL  THOSE   SCIENTIFICALLY 

INTERESTED   IN   THE   PRACTICES 

OF   OCULAR   REFRACTION 


Copyright  1921 

by  the 
CLEVELAND  PRESS 


TRANSMISSION  OF  RADIANT  ENERGY 

BY 

OPHTHALMIC  GLASSES 


TABLE  OF  CONTENTS 


CHAPTER  I.    RADIANT  ENERGY 

Nature  and  Distribution — Wavelengths  of  Light — Spectrum  of  Radia- 
tion— Radiation  and  Light  Sensations .' Pages  1-9 

CHAPTER  II.  COMMON  METHODS  OF  PRODUCING  AND  INVESTI- 
GATING ULTRAVIOLET,  VISIBLE  AND  INFRA-RED  RADI- 
ATIONS 

Spectrographs — Infra-red  Spectrum — Experimental  Apparatus  for  Infra- 
red Transmission  of  Glass — Spectra  of  Illuminants Pages  9-23 

CHAPTER  III.  TRANSMISSION  AND  ABSORPTION  OF  GLASS  FOR 
ULTRAVIOLET,  VISIBLE  AND  INFRA-RED  RADIATION 

Early  Work — Investigations  of  Crookes,  Coblentz,  Emerson,  Ham, 
Smith,  Luckiesh,  Gage  and  others — Transmission  in  the  Ultraviolet 
of  Various  Types  of  Glasses — Effect  of  Thickness  on  Transmission  of 
Specified  Glasses — The  Infra-red:  Early  Experiments — Transmission  of 
Various  Types  of  Glass  for  Infra-red Pages  23-67 

CHAPTER  IV.     TRANSMISSION  OF  THE  OCULAR  MEDIA 

Ultraviolet  and  Visible:  Investigations  of  Parsons,  Schanz,  Stock- 
hausen,  Birch-Hirschfeld,  Hallauer  and  others — The  Infra-red:  Inves- 
tigations of  Aschkinass,  Hartridge  and  Hill,  Luckiesh — Percentage 
Absorption  and  Transmission  of  the  Different  Ocular  Media — Energy 
Density  in  the  Eye Pages  67-82 


483432 


FOREWORD 


Through  the  courtesies  of  Dr.  Casey  A.  Wood,  Editor-in-chief  of  the 
American  Encyclopedia  of  Ophthalmology,  and  the  publisher,  Dr.  Geo. 
Henry  Cleveland,  of  the  Cleveland  Press,  the  writer  of  this  brochure 
has  been  able  to  secure  a  reprint  of  the  original  essay  for  the  use  of 
students  and  practitioners  who  may  be  interested  in  having  at  first 
hand  data  relative  to  the  transmission  of  radiant  energy  by  ophthalmic 
glasses. 

A  considerable  number  of  pages  have  been  devoted  to  a  discussion 
of  the  nature  and  distribution  of  radiant  energy  and  various  common 
methods  of  producing  and  investigating  ultraviolet,  visible  and  infra- 
red radiations,  in  order  that  the  full  significance  of  the  data  presented 
in  the  remainder  of  the  essay  may  be  disclosed. 

The  writer  claims  for  these  pages  nothing  more  than  an  attempt  to 
put  together  and  present  some  of  the  salient  points  with  reference  to 
a  subject  which  is  of  considerable  interest  and  importance.  Much  has 
yet  to  be  discovered  in  the  field  of  the  relations  between  radiant  energy 
and  the  eye. 

CHARLES  SHEARD 
Research  Division, 
American  Optical  Company, 
Southbridge,  Mass.,  1921. 


TRANSMISSION  OF  RADIANT  ENERGY 

BY 


CHAPTER   I.      RADIANT   ENERGY. 


We  know  that  all  about  us  energy  is  constantly  being  transferred 
from  one  place  to  another.  When  this  occurs  something  is  moving 
and  something  is  being  moved  through,  hence  there  must  be  a  me- 
dium of  transmission.  Radiation  is  a  form  or  kind  of  energy  and 
therefore  can  be  produced  from  other  forms  of  energy  and  can  be 
converted  into  other  kinds.  Heat  energy  is  a  convenient  form  for 
the  production  of  radiation.  For  instance,  the  resistance  offered  by 
the  filament  of  an  incandescent  lamp  to  the  flow  of  electric  current 
produces  energy  which  is  radiated  out  as  disturbances  in  the  ether. 
These  disturbances,  having  varying  wavelengths,  are  characterized 
as  heat  and  infra-red,  visible,  or  ultra-violet  light  when  they  are 
intercepted  and  received  by  a  suitable  body.  Heat,  for  example,  is 
not  radiant  energy  but  exists  only  when  it  ceases  to  be  radiation. 
And .  again,  radiations  of  the  proper  wavelengths,  when  received  by 


TRANSMISSION  OF  RADIANT  ENERGY 

the  eye,  cease  to  exist  as  radiant  energy  and  become  what  is  popu- 
larly spoken  of  as  light. 

Nature  and  Distribution  of  Radiant  Energy. 

Light  is  physically  defined  as  a  periodic  or  rhythmic  electromag- 
netic disturbance  in  a  transmitting  medium,  the  ether,  traveling  in 
the  form  of  transverse  waves  with  a  velocity  of  approximately  186,000 
miles  per  second.  At  first  glance  it  is  not  evident  that  there  is  any 
connection  between  light  and  electricity  (or  electric  waves).  Such 
a  relation  was  predicted  mathematically  by  Maxwell  at  about  1870. 
In  this  theory  the  assumption  was  made  that  light  waves  are  identical 
with  electromagnetic  disturbances  which  are  given  out  from  a  body 
in  which  electrical  oscillations  are  occurring.  Hertz  later  produced 
these  electric  waves  and  the  theory  of  Maxwell  was  given  an  experi- 
mental verification  and  the  science  of  wireless  telegraphy  and  tele- 
phony was  born.  The  oscillating  molecules,  atoms  and  their  elec- 
trons are  presumably  responsible  for  these  pulses  of  electromagnetic 
energy.  While  all  these  radiations  travel  in  free  space  with  the  same 
velocity,  they  differ  considerably  in  their  velocities  in  ordinary  media : 
in  glass,  for  example,  the  violet  rays  travel  less  rapidly  than  the  red 
rays.  All  of  these  rays  carry  energy;  that  is,  the  rays  are  the  actual 
physical  energy  in  transmission  and  hence  when  absorbed  will  pro- 
duce heat,  chemical  action  or  physiological  change.  Sometimes  the 
absorbed  radiation  is  not  converted  wholly  into  heat  but  enters  into 
chemical  reaction  or  is  changed  into  radiant  energy  of  wavelength 
differing  from  that  of  the  absorbed  energy.  Rays  shorter  than  the 
visible  and  known  as  ultra-violet  are  very  active  chemically,  affect 
photographic  films,  destroy  bacteria  and  animal  tissues  such  as  the 
outer  membrane  (the  conjunctiva)  of  the  eye:  they  also  cause  phos- 
phorescence and  fluorescence.  The  long  wavelengths  of  the  visible 
spectrum  are  received  by  the  normal  retina  and  interpreted  as  rep- 
resenting light  and  redness  of  color.  Beyond  the  longest  visible  red 
ray  (in  the  region  of  7600  Angstroms  or  tenth-meters)  come  the  infra- 
red rays.  These  are  commonly  spoken  of  as  the  heat  rays  because  of 
the  fact  that  the  energy  of  the  radiations  is  transformed  ordinarily 
in  the  maximum  percentages  into  heat. 

Wavelengths  of  Light. 

The  wavelength  of  light  can  be  measured  with  extremely  high  ac- 
curacy (vide  the  experiments  of  Michelson  with  the  interferometer 


TRANSMISSION  OF  RADIANT  ENERGY  3 

and  so  forth)  and  has  been  proposed  as  the  absolute  standard  of 
length  instead  of  the  meter,  which  was  intended  to  be  a  ten-millionth 
part  of  the  earth's  quadrant.  It  is  found,  however,  that  different 
kinds  of  radiant  energy  have  widely  different  wavelengths;  for  ex- 
ample, the  different  colors  (as  we  may  call  them)  of  light,  have  dif- 
ferent wavelengths,  red  light  having  the  longest  and  blue  the  shortest 
length  of  the  visible  region.  The  wavelength  may  be  specified  in 
units  known  as  the  micron  (p.),  the  tenth-meter  (t.  m.)  and  the 
Angstrom  (A).  The  micron  is  the  one-millionth  part  of  a  meter 
or  the  one-thousandth  part  of  a  millimeter.  The  Angstrom  unit  and 
the  tenth-meter  are  each  equal  to  one  ten-millionth  of  a  millimeter. 
As  a  result  a  radiation,  which  may  be  specified  as  red  light,  may  be 
defined  as  7000  A.,  7000  t.  m.  or  0.7  /x. 

Light  is,  therefore,  a  form  of  radiant  energy  which,  when  received 
by  the. eye,  gives  normally  the  sensations  of  sight  and  of  color.  The 
visible  spectrum  spans  one  octave  practically.  The  question  as  to 
the  sources  of  these  disturbances  in  the  ether  leads  us  to  a  brief  state- 
ment of  the  fact  that  atoms  are  now  known  to  be  composed  of  in- 
finitely small  particles  which  are  called  electrons.  These  electrons, 
under  given  physical  conditions,  oscillate  or  vibrate  with  certain 
definite  periods  or  frequencies.  The  number  of  electrons  composing 
the  atom  and  the  rate  of  frequency  of  vibration  depend  upon  the 
element,  or  the  so  called  atomic  number.  In  order  to  make  that  about 
which  we  are  writing  the  more  tangible,  imagine  a  carbon  arc  set-up 
and  ready  for  operation.  Before  the  arc  is  struck  there  is  no  light 
or  heat  and  no  changes  in  the  appearance  of  the  carbon.  As  the  arc 
is  "struck,"  electric  current  flows  through  the  carbons  and  they 
become  incandescent,  throwing  out  heat  and  light  and  other  forms 
of  energy.  By  virtue  of  the  disturbances  set  up  in  the  carbon  elec- 
trodes under  the  influence  of  an  electrical  potential  or  pressure,  the 
molecules  and  atoms  of  the  carbon  are  agitated :  the  current  passing 
through  the  carbon  heats  it  and  as  the  atoms  and  molecules  are  thrown 
into  vibration  and  absorb  energy  the  electrons  composing  the  atom 
become  correspondingly  disturbed,  vibrating  with  greater  frequencies 
and  by  their  vibrations  and  collisions  with  each  other  produce  dis- 
turbances in  the  ether.  These  disturbances  are  propagated  into  space 
by  wave  motion  and  since  the  disturbances  within  the  carbon  are 
very  complex,  a  corresponding  complex  emission  of  wave  energy  fol- 
lows. When  the  frequency  of  the  waves  approaches  400  million  mil- 
lions per  second  (4xl014  cycles  per  second)  a  dull  red  glow  ap- 
pears. When  the  frequencies  of  the  waves  range  between  400  and 
800  million  millions  per  second  (4xl014  to  8xl014  cycles)  the  sensa- 


TRANSMISSION  OF  RADIANT  ENERGY 

tion  produced  in  the  eye  is  that  of  white  or  nearly  white  dependent 
upon  the  percentages  of  the  various  wavelengths.  The  range  between 
400  and  800  million  millions  repesents  the  visual  spectrum  between 
violet  (800)  and  the  extreme  red  (roughly  400).  The  colors  of  the 
so  called  visible  spectrum  are  as  follows: 

Frequency.  Wave-length. 

Red 400  x  10  12  cycles  75  x  10  —  «  em. 

Orange    460  65 

Yellow    508  59 

Green    566  53 

Blue    652  48 

Indigo    710  45 

Violet    800  40 

*  '        * 

It  is  to  be  noted  at  this  point,  however,  that  the  actual  limits  of  the 
so-called  visual  spectrum  to  the  dark  adapted  eye  are  not  to  be  set  at 
0.75  n  and  0.4  /i  respectively. 

Spectrum  of  Radiation. 

The  spectrum  of  radiant  energy  extends  from  the  shortest  wave- 
length known,  that  of  the  X-ray  having  a  wavelength  of  the  order 
of  magnitude  of  lxlO~8  cm.  (one  hundred-millionth  part  of  a  centi- 
meter) to  the  longest  waves  due  to  the  fields  of  alternating  current 
circuits  and  having  wavelengths  approximating  fifteen  thousand  miles. 

Within  the  past  few  years  methods  of  crystal  spectrometry  as  de- 
vised by  Laue  and  elaborated  experimentally  by  the  two  Braggs  have 
led  to  determinations  of  the  wavelengths  of  X-rays.  Shaw  has  made 
measurements  upon  the  y  rays  given  out  by  radioactive  substances 
and  has  found  them  from  ten  to  one  hundred  times  shorter  in  length 
than  are  the  hardest  Roentgen  radiations.  Several  octaves  (an  octave 
representing  a  doubling  of  the  frequency)  are  missing  between  the 
longest  X-ray  and  the  shortest  so  called  ultra-violet  wavelength  which 
was  set  a  year  or  so  ago  by  Lyman,  of  Harvard,  at  about  0.06  p.  or 
something  less  than  0.1  /x.  Just  as  these  words  are  being  penned, 
however,  comes  the  report  that  Millikan,  of  Chicago,  has  succeeded 
in  getting  ultra-violet  waves  about  ten  times  shorter  than  the  shortest 
obtained  by  Lyman.  The  greater  part  of  the  research  conducted  in 
this  region  has  been  done  by  means  of  photography  carried  on  with 
spectroscopes  and  specially  sensitive  plates  enclosed  in  vacuum  cham- 
bers. Ordinary  crown  and  flint  lenses  and  prisms  cannot  be  used  in 
such  experimentation,  for  they  absorb  up  to  wavelength  0.30  p  as 
about  the  lowest  limit.  Quartz  lenses  and  prisms  are  therefore  used. 
In  working  with  the  spectrum  below  0.185  p.  such  effects  as  absorp- 


TRANSMISSION  OF  RADIANT  ENERGY  5 

tion  by  the  quartz  and  absorption  by  the  gelatine  of  the  photographic 
films  begin  to  exert  their  influence. 

The  actual  division  line  between  the  ultra-violet  and  the  visible 
spectra  is  commonly  and  somewhat  arbitrarily  set  at  0.4  p..  Doubtless 
this  division  point  has  arisen  because  of  the  various  determinations 
of  the  "visibility  curves"  of  eyes.  Hyde,  Cady,  Forsythe,  Hartwell, 
Nutting,  Ives,  Reeves,  Coblentz  and  others  have  investigated  these 
visibility  curves  with  considerable  care:  something  in  the  neighbor- 
hood of  0.4  p.  in  the  violet  is  about  the  limit  of  accurate  investigation 
with  present  experimental  photometric  devices.  These  visibility 
curves  are  not  to  be  confused  with  the  shortest  or  longest  wavelengths 
per  se  which  a  dark-adapted  eye  can  see  and  measure.  Certain  data  to 
be  presented  later  indicate  the  transmission  of  radiant  energy  through 
the  ocular  media  to  the  retina  having  wavelengths  considerably  shorter 
than  0.4  p..  There  are  doubtless  crystalline  lenses  which  absorb  all 
wavelengths  shorter  than  0.4  p.:  but  Hallauer,  for  instance,  found 
many  who  could  see  as  low  as  0.36  p.  to  0.37  p.  and  claims  that  in  the 
case  of  youthful  lenses  there  is  an  actual  transmission  of  rays  of 
wavelength  0.31  p.  to  0.33  p..  At  the  time  these  words  are  being 
written  a  careful  and  detailed  experimental  investigation  is  being  con- 
ducted by  the  writer  and  his  colleagues  upon  the  lowest  limit  of  radia- 
tion visible  to  a  dark-adapted  eye.  Wavelength  0.34  ju,  (with  definite 
and  yet  very  faint  violet  color)  has  been  readily  observed  by  several 
and  0.32  p.  under  proper  conditions  of  intensity.  It  does  not  seem 
logical,  therefore,  to  set  the  limit  of  visible  radiation  as  high  as  0.4  p.. 
We  shall,  however,  until  the  weight  of  evidence  is  to  the  contrary, 
follow  the  arbitrary  division  already  made,  although  in  certain  curves 
(vide  Figs.  27-35)  WTC  have  set  the  region  at  about  0.37  to  0.38  p.. 

The  visible  spectrum  covers  about  an  octave  and  extends,  roughly, 
from  0.38  p.  to  0.8  p..  It  comprises  a  very  small  portion  of  all  the 
known  spectrum.  The  highest  sensitivity  of  the  eye  is  in  the  yellow- 
green  region  at  0.56  p,  practically.  The  extreme  visible  red  is  at  about 
0.79  p..  Hyde  and  his  collaborators  have  investigated  the  "visibility" 
curves  in  this  region  (red  end)  with  considerable  thoroughness. 

The  red  end  of  the  spectrum  is  at  the  beginning  of  the  ultra-red 
or  infra-red  radiations.  These  rays  are  often,  but  improperly,  spoken 
of  as  heat  rays,  for  they  should  properly  be  classed  and  spoken  of  in  the 
same  manner  as  are  the  ultra-violet  and  the  visible.  In  1800  Sir  Wil- 
liam Herschel  found  that  when  a  thermometer  with  a  blackened  bulb 
was  moved  into  the  spectral  region  just  beyond  the  red,  there  was  a  rise 
in  temperature  indicated.  This  proved  that  there  were  radiations 


TRANSMISSION  OF  RADIANT  ENERGY 


beyond  the  limit  of  visual  sensitiveness.  Sir  William  Abney  suc- 
ceeded in  photographing  the  infra-red  spectrum  out  to  1.1  p.  with 
specially  prepared  plates.  The  late  Professor  Langley,  of  fame  in 
these  days  for  his  experimentation  upon  submarines  andv  aeroplanes, 
constructed  an  instrument  known  as  a  bolometer.  In  this  instru- 
ment, based  upon  the  Wheatstone  bridge,  one  arm  consists  of  a  fine 
blackened  platinum  wire  or  grid.  When  this  receives  radiation  it 
absorbs  it  and  the  temperature  is  raised,  the  resistance  of  the  wire 
changed  and  a  current  produced  in  a  detecting  instrument,  a  gal- 
vanometer. Langley  plotted  the  spectrum  to  about  61  p..  By  the  meth- 
ods of  ' '  reststrahlen, "  interference  and  focal  isolation  various  ex- 
perimenters, including  such  names  as  Rubens,  Hollnagel,  Nichols, 
Trowbridge,  and  Wood  and  his  co-workers,  have  succeeded  in  extend- 
ing the  investigations  step  by  step  into  the  infra-red  region  to  be- 


CINQ' 

I'l 


ELECT 

SURG 
CILLATIONS, 
GRO 


GHT 


VES 


SOUND  WAVES 


Fig.  1 — The  spectrum  of  radiation.     (From  Steimnetz:     Radiation,  Light  and 

Illumination.) 

tween  200  and  300  p..  This  corresponds  to  a  wavelength  of  about  0.2 
to  0.3  mm.  Rubens  and  von  Baeyer  in  1911  found  a  maximum  in 
the  long  wave  radiation  from  a  quartz  mercury  arc  at  343  p.. 

About  four  octaves  gap  exists  between  the  longest  infra-red  radia- 
tion as  detected  by  the  method  of  focal  isolation  and  the  shortest 
Hertzian  or  electric  wave  thus  far  found  by  von  Baeyer  and  having 
a  length  of  about  two  millimeters.  In  passing  from  one  set  of  radia- 
tions to  the  other  we  are  passing  from  the  region  where  the  molecule 
constitutes  the  minimum  sized  vibrator  to  that  in  which  molar  rela- 
tions hold,  for  electric  and  Hertzian  waves  are  produced  by  discharges 
between  electrodes.  High  frequency  currents,  surges  and  oscillations, 
arcing,  wireless,  lightning  phenomena  and  so  forth,  have  wavelengths 
ranging  from  the  limiting  wavelength  of  a  fraction  of  a  centimeter 
as  determined  by  von  Baeyer  up  to  4,000,000,000  p.  in  length  or,  in 
other  words,  miles  in  length.  Alternating  current  fields  have  cycles 
varying  from  15  to  133.  Such  figures  give  us  as  wavelengths  some- 
thing of  the  order  of  13,000  miles  and  1400  miles  respectively. 

Figure  1  is  a  graphical  reproduction  from  Steinmetz  (Radiation, 


TRANSMISSION  OF  RADIANT  ENERGY 


Light  and  Illumination,  page  18).  It  is  not,  at  some  points,  up  to  date 
in  its  representation,  but  it  serves  in  a  very  satisfactory  manner  to 
graphically  illustrate  the  distribution  of  radiant  energy  from  alter- 
nating currents  to  X  rays. 

Radiation  and  Light  Sensation. 

The  distribution  of  the  energy  among  the  different  wavelengths 
given  out  by  an  incandescent  solid  is  shown  in  Figure  2.  This  curve 
is  known  as  a  radiation  curve  and  is  the  envelop  of  the  plotted  values 
of  the  energy  over  a  great  range  of  wavelengths.  A  continuous  spec- 
trum, in  contradistinction  to  a  line  spectrum  such  as  is  given  by  a 
mercury  arc  for  example,  is  characteristic  of  the  radiation  from  solid 
bodies.  In  the  curve  of  Figure  2,  the  height  of  the  curve  at  any 
point  above  the  base  line  is  a  measure  of  the  relative  amount  of 


V          R  WAVE  LENGTH 

Pig.  2 — Radiation  curve  of  an  incandescent  solid.      (Courtesy  of  M.  Luckiesh.) 

energy  possessed.  The  amounts  of  energy  for  various  wavelengths 
are  by  no  means  equal.  The  region  to  which  the  eye  responds  or  is 
"tuned"  lies  between  V  (violet)  and  R  (red).  This  region,  follow- 
ing the  diagram  taken  from  Luckiesh  (Color  and  Its  Applications, 
page  8),  is  exaggerated  in  extent  for  the  sake  of  clearness. 

Figure  3,  copied  from  the  work  of  Langley,  shows  the  relative  dis- 
tribution of  energy  in  the  spectra  of  the  gas  flame,  the  electric  arc, 
the  solar  spectrum  and  the  fire-fly. 

As  the  temperature  of  an  incandescent  body  is  increased,  the  energy 
contained  in  the  shorter  wavelengths  increases  more  rapidly  than  the 
energy  in  the  longer  wavelengths.  In  the  visible  spectrum  the  violet 
and  the  adjacent  rays  increase  in  intensity  more  rapidly  with  increase 
of  temperature  than  does  the  red  end.  This  causes  the  light  emitted 
by  an  incandescent  solid  to  become  bluer  in  color  (or  less  red,  since 
the  redness  is  the  noticeable  feature)  as  the  temperature  is  increased. 
The  effect  of  raising  the  temperature  on  the  distribution  of  radiant 


TRANSMISSION  OF  RADIANT  ENERGY 

energy  given  out  by  an  incandescent  solid  is  shown  in  the  curves  given 
in  Figure  4.  The  numbers  on  the  curves  indicate  the  absolute  black- 
body  temperatures  (i.  e.  above  —273°  C,  since  0°  0=273  K).  The 


Pour  Curves  of  Equal  Areas,  showing  one  unit  of  heat  displayed 
successively  in  heat  spectrum  of  Gas.  Electric  Arc  Sun  and 
Pire-Ply. 

ABSCtSSAE.-WAVE  LENGTHS. 

OROINATES.- ENERGY  AS  HCAT. 

ENERGY  CURVE. 
Gas  Flame  Spectrum. 

MAXIMUM  AT  !?6. 


ENERGY  CURVE. 

Electric  Arc  Spectrum. 

MAXIMUM  AT  1*16. 


ENERGY  CURVE. 
Solar  Spectrum. 

MAXIMUM  AT  0*62 


ENERGY  CURVE. 
Fire-Fly  Spectrum. 

MAXIMUM  AT  0*57 


t»       it        ZA       2*        I*       3.0 


Fig.  3 — Distribution  of  energy  in  spectra  of  gas  flame,  electric  arc,  sun  and 

fire-fly. 


wavelengths  are  in  /*;  the  rays  to  which  the  eyes  are  sensitive  are 
enclosed  between  V  and  R.  Thus,  as  the  temperature  is  raised,  the 
maximum  of  the  radiation  curve  shifts  toward  the  shorter  wavelengths. 


TRANSMISSION  OF  RADIANT  ENERGY  9 

The  energy  distribution  curve  for  sunlight  (Figure  3)  shows  that 
the  maximum  lies  in  the  visible  region.  This  has  brought  forward 
the  hypothesis  that  the  eye,  being  as  we  know  it  to  be  the  product 
of  the  processes  of  evolution,  has  become  most  sensitive  to  the  rays 
of  such  wavelengths  as  are  in  maximum  percentages  in  the  radiation 
from  the  sun.  As  the  maximum  of  the  radiation  curve  shifts  toward 
the  shorter  wavelengths,  a  greater  proportion  of  the  energy  is  found 
in  the  visible  region  and  this  accounts  for  the  increased  .luminous 
efficiency.  All  tendencies  in  light  production  are  to  the  end  of  the 
development  of  materials  and  methods  which  will  enable  the  sources 


Fig.   4 — Showing  the   effect   of  temperature   on  the   radiation   from   an   incan- 
descent solid   (black  body).     (Courtesy  of  M.  Luckiesh.) 

to  be  operated  at  higher  temperatures.  This  is  the  advantage  of  the 
tungsten  filament  over  the  carbon  filament  lamp.  The  ideal  source 
from  the  visual  standpoint  would  be  one  which  affords  a  continuous 
spectrum  extending  only  from  the  blue  to  the  orange  roughly.  The 
distribution  of  energy  in  the  spectrum  given  out  by  the  fire-fly  ap- 
proaches very  closely  this  ideal.  Langley  and  Coblentz  have  shown 
that  ninety-five  per  cent,  is  available  as  luminous  energy. 


CHAPTER  II.      COMMON   METHODS  OF  PRODUCING   AND  INVESTIGATING 
ULTRA-VIOLET,    VISIBLE    AND    INFRA-RED    RADIATIONS. 

Spectrographs. 

Spectrographs  and  spectrometers  are  the  instruments  commonly 
used  in  investigations  upon  those  regions  of  the  ultra-violet,  visible  and 
infra-red  which  are  of  interest  to  us  either  from  the  standpoint  of 


10 


TRANSMISSION  OF  RADIANT  ENERGY 


cu 


1 


tib 


TRANSMISSION  OF  RADIANT  ENERGY  11 

the  eye  or  of  ophthalmic  glass  transmission."  One  of  the 'modern  forms 
of  spectrograph  is  shown  in  Figure  5  and  with  the  use  of  this  diagram 
the  essential  features  of  this  prism  spectrograph  will  be  pointed  out. 
The  three  essential  parts  of  such  instruments  are  the  collimator,  the 
prism  system  and  the  photographic  apparatus  or  telescope  which  may 
ta*ke  its  place  in  visual  work.  The  source  of  light  is  placed  before 
a  narrow  slit  S  in  the  collimator  tube,  or  the  light  from  the  sun  or 
an  electric  arc  may  be  concentrated  by  means  of  a  lens  upon  the  slit. 
At  the  other  end  of  this  tube  is  placed  an  achromatic  lens  system  and 
the  slit  and  lens  are  so  adjusted  that  parallel  light  will  fall  upon 
the  prism  P.  In  passing  through  the  prism  the  light  suffers  disper- 
sion and  as  a  result  there  emerges  a  parallel  beam  of  red  light  and 
a  parallel  beam  of  violet  light  with  beams  of  the  other  wavelengths 
situated  between  them.  The  pencil  of  red  light  is  brought  to  a  focus 
upon  the  photographic  plate  A  by  means  of  an  achromatic  lens  sys- 
tem in  the  telescopic  tube :  the  violet  rays  as  well  as  the  remainder  of 
the  visible  spectrum  are  likewise  focused.  Since  the  dispersion  for 
these  rays  is  different  a  spectrum,  extending  from  red  to  violet,  will 
be  found  upon  the  plate  or  visually  obtained  at  the  eye-piece  of  the 
telescope.  In  order  that  wavelengths  may  be  determined  the  prism 
must  be  calibrated.  One  of  the  latest  makes  of  spectrometers  car- 
ries the  constant  deviation  prism  and  is  provided  with  a  drum  device, 
the  barrel  of  which  is  calibrated  in  wavelengths.  Such  an  instru- 
ment as  this,  or  of  a  similar  character,  carrying  a  glass  prism  and 
glass  lenses  permits  of  the  making  of  photographic  or  ocular  observa- 
tions of  the  visible  spectrum.  When,  however,  it  is  desired  that 
investigations  be  carried  on  in  the  ultra-violet,  all  lenses  and  prisms 
must  be  of  quartz,  since  this  substance  does  not  absorb  the  ultra-violet 
until  the  limit  of  about  1800  Angstroms  is  reached. 

Other  types  of  instruments  employ  the  principle  of  auto-collima- 
tion.'  By  this  method  the  collimator  and  camera  lenses  are  entirely 
suppressed,  the  only  optical  member  being  the  prism  itself.  The 
employment  of  the  principle  of  auto-collimation  with  a  30°  prism 
simultaneously  shortens  the  apparatus,  simplifies  the  lens  system  and 
avoids  trouble  due  to  the  rotatory  properties  of  quartz,  since  the 
prism  is  traversed  twice  in  opposite  directions.  The  necessary  con- 
dition for  a  pure  spectrum  is  that  all  incident  and  refracted  rays 
shall  make  the  same  angle  with  the  refracting  surface.  This  is  ac- 
complished by  giving  the  front  and  back  surfaces  of  the  prism  P 
(Figure  6)  suitable  spherical  curvatures. 

An  excellent  instrument  to  use  for  the  photographic  examination 
of  the  ultra-violet  end  of  the  spectrum  is  the  Fery  quartz  spectro- 


12  TRANSMISSION  OF  RADIANT  ENERGY 

graph.  This 'instrument  is  shown  diagrammatically  in  Figure  6.  The 
quartz  prism  P  is  silvered  on  the  back  and  is  ground  in  such  a  way 
.  that  radiation  received  from  a  source  in  front  of  the  slit  at  A  is 
brought  to  a  focus  at  E  after  reflection  from  the  back  of  the  prism. 
Since  the  prism  in  this  instrument  performs  the  functions  of  both 
the  lenses  and  the  prism  in  an  ordinary  spectrograph,  there  are  no 
losses  due  to  absorption  other  than  those  which  occur  in  the  prism. 

The  matter  of  a  satisfactory  source  of  illumination  for  work  in 
the  ultra-violet  region  is  worthy  of  more  than  a  passing  remark.  Iron 
arcs  and  similar  devices  are  fairly  rich  in  ultra-violet  but  are  not 
continuous.  The  iron  arc  spectrum  on  direct  current  is  redundant 
in  lines;  the  same  arc  with  condensers  used  across  the  arc  affords  a 
satisfactory  source  and  a  very  uniformly  and  continuously  burning 


Fig.  6 — The  Fery  quartz  spectrometer. 

one.  A  condensed  spark  discharge,  using  an  induction  coil,  one  elec- 
trode being  made  of  iron  and  the  other  of  an  alloy  of  cadmium, 
aluminum,  magnesium  and  zinc  gives  a  most  excellent  spectrum  for 
work  in  the  ultra-violet ;  while  it  is  fundamentally  a  line  spectrum  it 
has  superposed  upon  it  rather  extensive  portions  of  a  continuous 
spectrum  down  to  about  2300  t.  m. 

Another  device  used  by  some  investigators  upon  the  subject  of 
glasses  for  protecting  the  eyes  consists  of  reflecting  the  light  from 
a  mercury  arc  from  the  face  of  a  magnesia  block.  The  spectrum  is, 
of  course,  discontinuous.  In  its  use,  however,  a  spectrum  is  formed 
by  a  small  quartz  spectrograph,  the  slit  being  wide  enough  to  furnish 
bands  from  each  ultra-violet  line  of  sufficient  width  for  photometer- 
ings.  A  series  of  exposures  of  equal  length  but  with  different  known 
illuminations  of  the  magnesia  surface  can  be  made  and  a  compari- 
son with  the  transmission  through  the  media  under  examination  car- 
ried out.  The  photographs  as  thus  made  can  be  measured  for  density 
on  a  polarization  photometer  or  other  device  of  a  similar  character 
and  curves  can  be  plotted  showing  the  connection  between  the  density 
and  illumination  for  each  line.  It  is  thus  possible  to  determine  the 


TRANSMISSION  OF  RADIANT  ENERGY  13 

transmission  of  glass  for  each  wavelength  obtainable  from  the  source 
of  illumination. 

Perhaps  the  best  device  for  producing  a  continuous  spectrum  is 
that  consisting  of  two  electrodes — aluminum  or  brass,  for  example — 
under  water  and  actuated  by  high  frequency.  A  wireless  oscillation 
transformer  as  a  source  of  excitation  has  been  found  very  satisfac- 
tory. There  is  less  trouble  with  the  question  of  purity  of  the  water 
used  but  it  does  not  afford  quite  as  continuous  a  spectrum.  The  limit 
of  continuity  is,  however,  about  2100  t.  m. ;  there  is  a  gradual  fall- 
ing off  in  intensity  of  the  rays  in  the  extreme  ultra-violet.  Details 
in  this  matter  are  given  by  Howe  in  a  paper  on  a  Photometric  Method 
of  Measuring  Ultra-violet  Absorption  (Pliys.  Review,  Ser.  II,  Vol. 
8,  1916). 

• 

Infra-red  Spectrum,. 

Herschel  in  1800  demonstrated  the  existence  of  a  portion  of  the 
spectrum  beyond  the  extreme  visible  red.  He  did  this  by  means  of 
a  thermometer  with  a  blackened  bulb.  A  rise  in  temperature  was 
indicated  in  the  region  just  beyond  the  red.  In  the  study  of  infra- 
red rays  we  need  a  prism  made  of  a  substance  which  does  not  absorb 
radiations  of  long  wavelength  and,  in  the  second  place,  we  need  a 
device  or  receiving  instrument  which  will  indicate  a  very  small  rise 
of  temperature  due  to  the  radiations  absorbed.  Prisms  of  rock-salt 
(or  fluor  spar:  not  now  used  much  because  of  the  scarcity)  are  em- 
ployed. The  instruments  used  for  absorbing  the  radiations  and  in- 
dicating the  consequent  rise  in  temperature  are  the  thermopile,  the 
radio-micrometer  and  the  bolometer. 

Dr.  Langley  of  the  Smithsonian  Institute  carried  on  a  very  elab- 
orate study  of  the  infra-red  solar  spectrum  making  use  of  his  bolo- 
meter. This  instrument  consists  of  blackened  strips  of  platinum, 
about  a  tenth  millimeter  in  breadth  and  a  hundredth  millimeter  in 
thickness,  arranged  to  form  two  arms  of  a  Wheatstone  bridge.  When 
the  usual  galvanometer  and  battery  connections  are  made,  the  resist- 
ances in  the  remaining  arms  are  adjusted  so  that  the  galvanometer 
shows  no  deflection.  When  radiation  falls  upon  one  arm  of  the  bridge, 
however,  the  balance  of  the  bridge  is  destroyed  and  a  deflection  of 
the  galvanometer  follows.  This  galvanometer  deflection  affords  a 
means  of  relative  measurement  of  the  absorption  of  energy.  The 
sensitiveness  of  the  instrument  is  such  that  a  rise  of  temperature 
amounting  to  not  more  than  one  hundred-millionth  of  a  degree  Centi- 
grade can  produce  a  measurable  deflection.  "What  would  be  a  dark 


14  TRANSMISSION  OF  RADIANT  ENERGY 

band  in  the  spectrum,  could  our  eyes  be  affected  by  the  long  infra- 
red waves,  will  fail  to  heat  the  platinum  strip  and  the  galvanometer 
deflection  will  be  diminished  or  reduced  to  zero"  (Edser.  Light, 
page  345).  By  means  of  this  bolometric  device  Langley  investigated 
the  infra-red  through  a  region  extending  from  0.76  p.  (7600  t.  m.) 
to  5.3  11  (53,000  t.  m.). 

In  1880  Sir  William  Abney  obtained  through  the  use  of  specially 
prepared  plates  a  photographic  record  out  to  1.1  /*.  The  special 
character  of  plates  and  the  difficulty  of  handling  photographic  work 
in  regions  sensitive  to  the  red,  together  with  the  low  limit  of  the 
radiations  recorded,  make  this  method  practically  useless  for  experi- 
mental work. 

Following  Langley,  a  series  of  most  valuable  investigations  has  been 
carried  out  by  Kubens,  Nichols,  Aschkinass,  Wood,  von  Baeyer  and 
others  working  in  this  region.  By  means  of  the  radiometer,  the  method 
of  "  reststrahlen "  and  focal  isolation  the  investigations  in  the  infra- 
red have  been  gradually  extended  to  about  343  /t  or  roughly  one- 
third  of  a  millimeter, 
i 

Experimental  Apparatus  for  Infra-red  Transmission  of   Glass. 

The  refinements  and  the  extended  ranges  of  wavelength  obtain- 
able by  several  of  the  methods  just  mentioned  are  not  -necessary  in 
investigating  the  absorption  and  transmission  of  the  ocular  media 
and  of  glass  in  the  infra-red  region.  Tests  by  a  considerable  number 
of  investigators  show  that  the  transmission  of  the  eye  media  becomes 
very  small  after  the  region  3.5  /*  is  passed,  while  the  transmission  of 
ordinary  glass  drops,  in  general,  to  the  order  of  10  to  15  per  cent, 
at  4.5  p.. 

A  quite  satisfactory  form  of  instrument  for  the  examination  of 
the  transmission  of  glasses  and  similar  media  in  the  infra-red  region 
is  the  Hilger  infra-red  spectrometer.     The  essentials  of  this  instru 
ment  are  shown  diagrammatically  in  Figure  7.    In  Figure  8  is  given 
a  photographic  reproduction  of  this  instrument. 

Kadiation  from  a  suitable  source  is  allowed  to  pass  through  a 
narrow  slit  8  of  the  order  of  magnitude  of  one-hundredth  of  an  inch 
in  width.  The  radiation  from  8  is  received  by  a  concave  mirror  K 
by  which  it  is  collimated.  It  then  falls  on  the  rock-salt  prism  P,  by 
which  it  is  dispersed  and  received  on  the  plane  mirror  M  and  reflected 
to  the  concave  mirror  R.  This  concave  mirror  R  then  focusses  the 
radiation  on  the  slit  T,  behind  which  is  mounted  a  Hilger  bismuth- 
silver  thermopile  which  acts  as  the  receiving  instrument.  The  mirrors 


TRANSMISSION  OF  RADIANT  ENERGY  15 

are  made  of  nickeled  steel.  The  slit  at  T  is  of  the  order  of  one  one- 
hundredth  inch  in  width.  The  prism  P  and  the  mirror  M  are  mounted 
on  a  table  which  can  be  rotated  around  a  vertical  axis  by  means  of  a 
fine  screw  which  is  attached  to  a  calibrated  drumhead.  From  this 
drumhead  the  wavelength  used  for  experimental  purposes  can  be 
read  directly.  By  this  rotation  of  the  prism  P  any  desired  part  of 


T 

Fig.  7 — Essentials  of  construction  of  the  Hilger  infra-red  spectrometer. 

the  spectrum  can  be  made  to  fall  upon  the  slit  T.  The  thermopile  and 
the  whole  instrument  must  be  carefully  protected  from  external 
radiation. 

The  thermopile  serves  as  the  receiving  instrument  in  this  type  of 
instrument.  Nobili  devised  what  he  called  a  ' '  pile  "  or  a  form  of  thermo- 
electric battery  in  which  there  are  a  large  number  of  elements  in  a 
very  small  space.  For  this  purpose  he  joined  the  couples  of  bismuth 
and  antimony  in  such  a  manner  that,  after  having  formed  a  series  of 
five  couples  as  shown  in  Figure  9  (B)  the  bismuth  from  &  was  soldered 
to  the  antimony  of  the  second  .series  similarly  arranged;  the  last  bis- 


16  TRANSMISSION  OF  RADIANT  ENERGY 

muth  of  this  to  the  antimony  of  the  third  and  so  on.  The  whole  pile 
thus  consisted  of  a  number  of  bismuth-antimony  couples.  The  couples 
can  be  insulated  from  each  other  by  means  of  small  paper  bands 
covered  with  varnish  and  are  then  enclosed  in  a  suitable  frame  P 
(Figure  9 A)  so  that  the  only  solderings  appear  at  the  two  ends  of 
the  pile.  Two  small  binding  posts,  m  and  n,  insulated  in  an  ivory  ring, 
communicate  in  the  interior,  one  with  the  antimony,  representing  the 
positive  pole,  and  the  other  with  the  last  bismuth,  representing  the 


Fig.  8 — The  Hilger  infra-red  spectrometer. 

negative  pole.  These  terminal  points  then  connect  with»a  galvano- 
meter :  this  instrument  detects  the  thermo-electric  current.  The  action 
of  the  thermopile  depends  upon  the  principle  that  if  one  set  of  junc- 
tions is  at  a  higher  temperature  than  the  second  set  an  electric  current 
is  produced.  If  these  thermo-couples  are  made  of  the  proper  elements 
and  are  connected  in  the  circuit  of  a  sensitive  galvanometer,  extremely 
small  fractions  of  a  degree  rise  in  temperature  can  be  detected.  The 
electric  current  arises  in  every  case,  however,  because  of  the  difference 
in  temperature  of  the  two  faces  of  the  thenno- junctions. 

A  sufficiently  sensitive  galvanometer  is  used  as  the  instrument  for 
the  detection  of  the  current.    The  strength  of  the  current  is  propor- 


TRANSMISSION  OF  RADIANT  ENERGY  17 

tional  to  the  mirror  deflection.     This  deflection  can  be  measured  by 
means  of  a  lamp  and  scale. 

For  investigations  in  the  infra-red  transmission  of  glass  it  is  found 
that  the  Nernst  glower  is  a  very  satisfactory  source.  The  distribution 
of  energy  in  wavelengths  of  the  emission  of  a  Nernst  glower  varies 
considerably  with  the  temperature.  The  radiation  from  such  a  glower 
is  characterized  by  two  maxima  at  about  2.5  ju,  and  5.5  to  6  /x.  At  low 
temperatures  (2  watts  to  7  watts)  the  latter  maximum  (5.5  /x)  is  the 
more  prominent.  As  the  temperature  is  raised  (11  or  more  watts) 
the  maximum  of  the  energy  distribution  appears  in  the  region  of 
about  2  /JL  (See  paper  by  Coblentz,  Bulletin  of  the  Bureau  of  Standards, 
Vol.  4,  1907.).  This  region  from  0.7  /x  to  3  /x  is  that  which  is  of 
interest  from  the  standpoint  of  transmission  and  absorption  in  the  eye 


Fig.   9 — The  thermopile. 

media  and  in  various  kinds  of  glass.  The  Nernst  glower  is,  therefore, 
used  in  such  experimental  work  with  the  maximum  energy  in  the 
region  of  1.5  to  2  p. 

Spectra  of  Illuminants. 

The  spectral  distribution  of  energy  in  the  radiation  from  different 
illuminants  is  of  great  importance  in  the  consideration  of  color.  This 
variation  in  the  spectral  character  of  illuminants  is  due  to  the  tempera- 
ture and  composition  of  the  radiating  body  and  also  to  the  state  in 
which  it  exists  when  giving  out  luminous  energy.  A  gaseous  body 
gives  out  only  certain  definite  rays  and  the  spectrum  is  said  to  be  a 
line  spectrum.  Quite  often  these  spectral  lines  are  crowded  together 
in  such  a  manner  as  to  give  to  the  spectrum  a  fluted!  or  banded 
appearance.  This  is  known  as  a  band  spectrum.  Also,  the  constancy 
of  the  spectrum  lines  given  by  any  substance  (element)  in  gaseous 
form  is  a  striking  feature.  For  example,  the  visible  spectrum  of  so- 
dium consists  of  a  double  line  (5890  t.  m.  and  5896  t.  m.)  and  whenever 
this  double  line  is  found  in  a  spectrum  it  is  certain  that  sodium  is 


18  TRANSMISSION  OF  RADIANT  ENERGY 


a.  Mercury  arc. 


b.  Helium. 


c.  Iron  arc. 


d.  Yellow  flame  arc. 


e.  Carbon  arc. 


f.  Carbon  arc. 


g.  Carbon  arc. 


h.  Tungsten  incandescent  lamp. 


i.  Skylight. 


j.  Skylight. 


Fig.  10 — Representative  spectra.     (Courtesy  of  M.  Luckiesh.) 


TRANSMISSION  OF  RADIANT  ENERGY 


19 


present  in  the  radiating  substances.  This  constancy  of  spectra  forms 
a  basis  of  analysis  more  sensitive  than  the  most  accurate  chemical  tests. 
The  element  helium  was  discovered  by  means  of  spectroscopy  some- 


0.68 


Fig.  11 — Distribution  of  energy  in  the  visible  spectrum  of  various  illuminants. 

Significance  of  letters  on  curves  given  in  Table   II. 

(Courtesy  of  M.  Luckiesh.) 

time  before  it  was  terrestrially  found.  The  vacuum  tube,  the  electric 
spark,  the  arc  and  the  flame  are  of  use  in  studying  the  spectra  of  ele- 
ments and  their  compounds. 

A  continuous  spectrum  is  emitted  by  an  incandescent  solid.     The 
spectrum  of  an  incandescent  electric  lamp,  for  example,  is  continuous. 


0  TRANSMISSION  OF  RADIANT  ENERGY 

The  energy  of  the  electric  current  running  through  such  a  filament 
is  converted  into  radiant  energy.  The  continuous  spectrum  is,  as  its 
name  signifies,  the  antithesis  of  the  line  spectrum :  or  it  may  be  con- 
sidered as  an  infinitely  numbered  line  spectrum.  There  are  no  breaks 
or  apertures  in  the  emission.  Sometimes  both  a  line  and  a  continuous 
spectrum  are  emitted  by  an  illuminant.  Such  a  condition  exists  in 
the  ordinary  electric  Carbon  arc.  The  center  of  the  arc  is  an  incan- 
descent solid  and  therefore  emits  visible  rays  of  all  wavelengths;  the 
incandescent  gas  of  the  arc  between  the  electrodes  emits  a  line 
spectrum  which  depends  as  to  its  appearance  upon  the  surrounding 
medium  and  the  character  of  the  carbon  electrodes.  In  Figure  10 
are  shown  several  representative  spectra  photographed  by  means  of 
a  sensitive  spectrograph  using  Cramer  spectrum  plates  which  were 
made  specially  sensitive  to  the  visible  rays  (Luckiesh:  Color  and  Its 
Applications,  page  17).  The  reproduced  spectrograms  contain  line 
spectra,  banded  spectra  and  continuous  spectra.  It  will  be  seen  that 
the  two  gases,  mercury  and  helium,  emit  line  spectra.  The  arcs  emit 
both  continuous  and  line  spectra.  The  relative  prominence  of  the  line 
spectra  depends  upon  the  relative  intensities  of  the  radiation  from  the 
arc  as  compared  with  that  from  the  solid  electrodes.  For  instance,  the 
line  spectrum  is  much  more  prominent  in  the  yellow  flame  arc  than 
in  the  ordinary  carbon  arc.  As  is  well  known,  the  arc  vapor  con- 
tributes a  much  greater  proportion  of  the  light  in  the  former  than  in 
the  latter  illuminating  source.  The  line  spectrum  of  carbon  is  sub- 
ject to  changes  because  of  the  character  and  amounts  of  impurities 
which  may  be  present  in  the  carbons.  The  three  spectra  of  the  carbon 
arc  given  in  Fig.  10  were  taken  within  a  few  minutes'  time  and  show 
these  variations.  (The  apparent  absorption  in  the  green  region  in  all 
these  photographs  is  due  to  lack  of  sensitiveness  of  the  plates  used  in 
the  green-blue  region).  The  tungsten  filament,  h,  is  seen  to  emit  a 
continuous  spectrum.  Two  spectrograms  of  light  from  the  sky  are 
shown  in  i  and  j  and  bring  out  (perhaps  rather  poorly)  the  presence 
of  narrow  black  absorption  lines.  The  solar  spectrum  is  of  interest 
particularly  because  of  the  fact  that  it  is  a  continuous  band  crossed 
by  many  fine  dark  lines.  These  lines  were  discovered  in  all  probability 
by  Wollaston  in  1802  but  were  studied  with  better  instruments  by 
Fraunhofer  in  1814  and  are  consequently  known  as  Fraunhofer  lines. 
These  absorption  lines  are  due  to  the  removal  of  the  corresponding 
radiations  by  the  vapors  in  the  solar  atmosphere.  The  chief  Fraun- 
hofer lines  with  their  wavelengths,  colors  and  sources  are  given  in 
Table  I. 


TRANSMISSION  OF  RADIANT  ENERGY 


TABLE   I. 

Principal  Fraunhofer  Lines. 

Line.  Wave-length.  Color. 

A    0.7594^  Red 

a    0.7185  Red 

B  0.6876  Red 

C   0.6563  Red 

D,    0.5896  Yellow 

D2 0.5890  Yellow 

E    0.5270  Green 

b,    0.5184  Green 

b2   0.5173  Green 

b4  0.5168  Green 

F  0.4861  Blue 

G   0.4308  '  Violet 

H 0.3969  Violet 

K    .  .   0.3934  Violet 


Source. 

Oxygen  in  atmosphere 
Water  vapor 
Oxygen  in  atmosphere 
Hydrogen  in  sun 
Sodium  in  sun 
Sodium  in  sun 
Calcium  in  sun 
Magnesium  in  sun 
Magnesium  in  sun 
Magnesium  in  sun 
Hydrogen  in  sun 
Calcium  in  sun 
Calcium  in  sun 
Calcium  in  sun 


Figure  11  gives  curves  showing  the  spectral  distribution  of  energy 
in  the  visible  region  for  various  illuminants.  These  data  were  obtained 
chiefly  by  Hyde,  Ives,  Cady  and  Luckiesh  working  in  the  Nela  Research 
Laboratory.  Table  II  gives  the  numerical  data  as  well  as  the  sig- 
nificance of  the  letters  attached  to  the  different  curves  (Luckiesh, 


TABLE  n. 
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TRANSMISSION  OF  RADIANT  ENERGY 


Color  and  Its  Applications,  page  21).  It  will  be  noted  that  all  curves 
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0.59  n  (approximately  sodium  D)  equals  one  hundred.  This  method 
of  plotting  gives  the  relative  distribution  of  energy  for  approximately 
the  same  amounts  of  total  light  sensation.  All  of  these  curves  show 


396.9     430.8 


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UE                    6REEN       YELlOW         ORANGE             RED                        INFRA-R£0. 
WAVE-LENGTH  ,  METERS  .  10'* 

Fig.  12 — Eelative  spectral  distribution  of  radiant  power  in  various  sources: 
-4=Hefner  lamp,  H.  E.  Ives,  Trans.  I.  E.  S.,  5,  p.  208,  1910;  _B=acetylene  flame, 
Coblentz  and  Emerson,  B.  S.,  13,  p.  363,  1916;  C= tungsten  (gas)  incandescent 
lamp  (No.,  1717)  at  15.6  lumens  per  watt,  data  by  Coblentz;  D— black  body  at 
5,000°  absolute — approximately  sunlight,  computed  from  Planck  equation;  E= 
blue  sky,  H.  E.  Ives,  Trans.  I.  E.  S.,  5,  p.  208,  1910 ;  fl#=Heraeus  quartz-mercury 
lamp,  81  watts,  W.  W.  Coblentz,  B.  S.,  9,  p.  97,  1913. 

Relative  visibility:  Lvz=Relative  visibility  curve  for  the  average  human  eye 
(or  luminosity  of  a  source  having  constant  radiant  power  throughout  the  visible 
spectrum),  Coblentz  and  Emerson,  B.  S.,  14,  p.  192,  1917. 

Relative  luminosity=relative  radiant  power  times  relative  visibility:  LA.=: 
luminosity  of  Hefner  lamp;  LE=luminosity  of  blue  sky. 

(From  Technologic  Paper  No.  119,  1919.  Permission  of  The  Bureau  of  Stand- 
ards.) 

that  all  artificial  light  sources,  such  as  the  'Welsbach,  acetylene  and 
tungsten  incandescent  lamps  are  relatively  rich  in  longer  visible  wave- 
lengths and  decidedly  deficient  in  terms  of  percentages  in  the  extreme 
blue  and  violet. 

Figure  12  is  taken  from  a  recent  paper  by  Gibson  and  McNicholas 
(The  Ultra-violet  and  Visible  Transmission  of  Eye-Protective  Glasses, 
Bulletin  of  Bureau  of  Standards,  June,  1919)  and  gives  data  by  Ives 


TRANSMISSION  OF  RADIANT  ENERGY  23 

and  by  Coblentz.    The  descriptive  matter  accompanying  the  diagram 
makes  clear  the  significance  of  the  curves. 

CHAPTER  III.     TRANSMISSION  AND  ABSORPTION  OF  GLASS  FOR  ULTRA-VIOLET, 
VISIBLE    AND    INFRA-RED    RADIATION. 

The  Ultra-violet  and  the  Visible. 

It  is  a  well  known  fact  that  ultra-violet  light  (light  of  wave-length 
less  than  3900  t.  m.  roughly)  may  exert  harmful  physiological  effects 
on  the  eye  and  skin,  but  just  how  much  of  this  deleterious  action  is  to 
be  ascribed  to  general  energy  radiation  and  how  much  to  specific  radia- 
tion is  a  matter  that  has  by  no  means  been  settled.  It  is  quite  generally 
agreed  that  the  extreme  ultra-violet  rays,  i.  e.,  those  of  wave-length 
3000  t.  m.  or  less,  cause  injury  when  in  sufficient  quantity  or  intensity. 
There  are  those  who  claim,  and  with  considerable  evidence  in  support 
thereof,  that  the  rays  between  3000  and  3600  t.  m.  also  cause  injury. 
Nutting  (Bureau  of  Standards,  Circular  No.  28)  believes  that  the 
3650  t.  m.  of  the  mercury  arc  contributes  about  80  per  cent,  of  the 
"fatigue  effect"  when  this  arc  is  used  as  a  source  of  light.  Whatever 
may  be  the  extent  of  the  extremely  harmful  regions  and  whatever 
opinions  may  be  held  as  to  what  radiations  are  or  are  not  harmful, 
there  are  still  many  industrial  processes  requiring  special  protection 
of  the  eyes,  and  the  excessive  amount  of  ultra-violet  or  of  infra-red 
radiation  may  be  and  generally  is  one  of  the  very  important  factors. 
It  is  therefore  a  matter  of  importance  to  know  much  of  the  physio- 
logical and  pathological  effects  of  radiation :  these  we  shall  consider  in 
another  part.  It  is  likewise  of  great  importance  that  we  should  know 
how  much  of  a  specified  radiation  gets  through  a  given  sample  of  glass 
or  other  absorbing  medium. 

In  1889  Widmark  (Skand.  Arch.  I,  264)  made  experiments  on  the 
subject  of  the  effects  of  ultra-violet  light  on  the  eyes  of  laboratory 
animals  and  reproduced  the  stages  of  electric  ophthalmia  in  rabbits' 
eyes.  Perhaps  stimulated  by  the  possibilities  of  the  protection  of  the 
eyes  suggested  by  Widmark 's  experiments,  Schulek  (Ungar.  Beitr.  z. 
AugenheilJc.  I,  101,  1895  and  2,  1899)  first  studied  the  means  of  pro- 
tecting the  eyes  against  ultra-violet  rays  and  found  that  certain  liquids 
had  the  highest  absorptive  powers  of  the  transparent  media  investi- 
gated. These  liquids  absorbed  all  rays  below  3960  t.  m.  He  suggested 
that  these  solutions  should  be  enclosed  in  flat,  oval-shaped  glass  cham- 
bers made  to  fit  the  eyes  and  to  protect  them  from  injuries  due  to  the 
ultra-violet  radiations. 

Stearkle  (Arch.  f.  Augenheilk.  50,  1904),  Vogt   (Arch.  f.  Augen-. 


24  TRANSMISSION  OF  RADIANT  ENERGY 

heilk.  59,  1907)  and  Hallauer  (Vers.  d.  Ophth.  Ges.  Heidelberg,  1907) 
studied  the  absorptive  properties  of  blue  uviol,  yellowish  and  smoky- 
gray  glasses.  The  last  named  worker  produced  by  a  secret  process 
the  so  called  Hallauerglas.  Following  a  like  study  Fieuzal  (Bull,  de  la 
din.  nat.  oph.  No.  3,  1885)  produced  a  glass  known  as  Fieuzalglass. 
Also  a  yellow-green  glass  patented  under  the  name  of  Enixanthosglas 
was  offered,  as  well  as  a  variety  of  modifications. 

In  1907  Schanz  and  Stockhausen  (Klin.  Moiiatsbl.  f.  Augenheilk. 
1907)  after  finding  that  electric  ophthalmia  could  be  produced  through 
18  mm.  of  common  glass,  began  to  study  the  problem  of  manufacturing 
a  colorless  glass  of  high  ultra-violet  absorptive  power.  In  1909  they 
produced  and  patented  a  glass  of  higher  absorptive  power  than  hard 
flint  and  named  it  Euphosglas.  This  glass  has  a  light,  yellowish-green 
tinge  and  fluoresces  in  ultra-violet  light. 

In  1909  Birch-Hirschfeld  (Zeitschr.  f.  Augenheilk.  21,  1909)  studied 
photometrically  the  absorptive  power  of  various  glasses  with  con- 
siderable accuracy.  At  about  the  same  time  Vogt  -(Arch.  f.  Augen- 
heilk. 59,  1907)  compared  a  new  and  very  hard  flint  glass  produced 
by  Schott  with  his  absorptive  solutions  and  found  that  it  had  about 
the  same  absorptive  efficiency,  beginning  at  4050  t.  m.  and  giving 
practically  complete  absorption  below  3960  t.  m.  Hallauer  (Arch.  f. 
Augenheilk.  54,  1909)  measured  photometrically  the  absorption  powers 
of  the  various  protective  glasses  then  available. 

As  the  number  of  kinds  of  glass  for  cutting  out  various  wavelengths 
became  greater  and  more  available,  the  question  arose  as  to  what 
spectral  range  constituted  the  best  illumination.  Voege  (Electro. 
Zeits.  No.  33,  1908)  maintained  that  the  light  from  the  clouds  or  clear 
sky  had  been  for  ages  a  normal  illumination  for  the  eyes  and  that 
it  contained  a  considerable  amount  of  ultra-violet  light  as  low  as  3000 
t.  m.  Hertel  and  Henker  (Arch.  f.  Ophthal.  73,  1910)  carried  out  a 
very  elaborate  set  of  experiments  in  the  Zeiss  Laboratory,  Jena,  and 
came  out  in  support  of  Voege.  These  experimenters  believed  that  the 
best  glass  is  one  that  will  reduce  the  spectrum  of  the  particular  light 
to  which  the  eyes  are  exposed  to  the  closest  possible  approximation  to 
the  spectrum  of  cloud  and  sky  lights.  For  observation  of  the  strongest 
arc  lights  at  close  range  they  considered  the  neutralglas  F  3815  of 
Schott  to  be  best,  claiming  that  this  glass,  in  layers  thinner  than  any 
other  glass,  may  be  used  to  observe  directly  a  bare  20  amperes  arc 
light  at  a  foot  and  a  half  without  injury,  since  the  spectrum  is  about 
the  same  as  that  of  cloud  light  minus  the  ultra-violet  portion.  Schanz 
and  Stockhausen  (Arch.  f.  Ophth.  75,  1919)  criticized  this  work,  par- 
ticularly on  the  basis  that  skylight  or  cloud  light  cannot  be  taken  as 


TRANSMISSION  OF  RADIANT  ENERGY  25 

the  ideal  light.  They  furthermore  cite  the  work  of  Handmann 
(Monatsbl.  f.  Augenk.  47,  1909)  showing  that  a  very  large  group  of 
cataracts  began  in  the  quadrant  of  the  lens  most  exposed  during  life 
to  the  light  of  the  sky.  It  may  be  stated,  in  brief,  that  during  the  past 
ten  years  these  various  discussions  and  opinions  have  caused  the  number 
of  protective  eye  glasses  for  general  wear  or  specific  purposes  to  be 
multiplied  and  attention  to  be  paid  to  glasses  affecting  the  ultra-violet, 
the  visible  and  the  infra-red. 

No  survey  of  the  development  of  protection  glasses  would  be  com- 
plete without  mention  of  the  paper  on  The  Preparation  of  Eye- 
Preserving  Glass  for  Spectacles  (Trans,  of  Roy.  Soc.,  1913)  delivered 
by  Sir  William  Crookes  before  the  Royal  Society  on  November  13th, 
1913.  Crookes  was  engaged  from  1909  to  1913  in  connection  with  the 
Glass  Workers  Cataract  Committee  of  the  Royal-  Society  and  experi- 
mented on  the  effect  of  adding  various  metallic  oxides  to  the  constitu- 
ents of  glass  in  order  to  cut  off  the  ultra-violet  and  the  infra-red  rays. 
The  main  object  of  the  researches  was  to  prepare  a  glass  which  would 
cut  off  the  rays  from  highly  heated  molten  glass  which  apparently 
damaged  the  eyes  of  workmen.  Photo-spectrographic  and  other  exami- 
nations were  made  of  the  radiation  emitted  from  the  molten  glass 
under  working  conditions.  Sir  Wm.  Crookes,  in  the  paper  referred 
to,  writes:  "Taking  the  ordinary  limit  of  visibility  to  lie  between 
3900  t.  m.  and  7600 1.  m.  it  is  seen  that  with  an  exposure  of  three  hours 
to  the  highest  heats  the  strength  of  impression  does  not  extend  much 
into  the  ultra-violet.  The  heat  rays  are  very  strong  and  if  injury  to 
the  eye  is  caused  by  exposure  to  radiation  from  the  molten  glass,  a 
protective  glass  should  be  opaque  to  infra-red  rays.  These  being 
present  in  the  radiation  from  molten  glass  in  far  greater  abundance 
than  the  ultra-violet  rays,  the  inference  is  that  it  is  to  the  heat  rays 
rather  than  to  the  ultra-violet  rays  that  glass  workers'  cataract  is  to 
be  ascribed.  It  is,  however,  certain  that  exposure  to  excess  of  ultra- 
violet light  also  injuriously  affects  the  eye.  That  the  ultra-violet  rays 
act  on  the  deeper-seated  portions  of  the  eye  is  shown  by  the  intense 
fluorescence  of  the  crystalline  lens  induced  by  these  rays.  Besides  the 
invisible  rays  at  each  end  of  the  spectrum,  the  purely  luminous  rays, 
if  present  in  abnormal  intensity,  are  found  to  damage  the  eye.  It, 
therefore,"  would  be  an  advantage  if  in  addition  the  obscuring  glass 
for  the  spectacles  were  to  be  of  a  neutral  or  gray  tint. ' ' 

In  discussing  the  results  of  his  experimental  work  Crookes  says  that 
the  first  necessity  is  to  find  a  glass  which  will  cut  off  as  much  as 
possible  of  the  heat  radiation.  "For  ordinary  use,"  he  writes,  "when 
no  special  protection  against  heat  radiation  is  needed,  the  choice  will 


26  TRANSMISSION  OF  RADIANT  ENERGY 

rest  on  whether  the  ultra-violet  or  the  luminous  are  most  to  be  guarded 
against;  or  whether  the  two  together  are  to  be  toned  down."  His 
experimental  work  gave  glasses  which  were  very  effective  in  cutting 
out  wavelengths  shorter  than  3700  t.  m.  The  colors  of  these  glasses 
were  pale-green,  yellow  and  neutral.  Likewise  glasses  of  much  trans- 
parency were  produced  which  transmitted  from  99.5  to  70  per  cent,  of 
the  incident  light.  The  choice  between  this  range  of  glasses  would 
depend  on  the  conditions  required.  Special  glasses  were  devised  which 
are  ' '  restful  to  the  eyes  in  the  glare  of  the  sun  on  chalk  cliffs,  expanses 
of  snow,  or  reflected  from  the  sea.  *  *  *  Moreover,  they  have  the 
advantage  of  cutting  off  practically  all  of  the  ultra-violet  rays  and  also 
a  considerable  amount  of  the  heat  radiation." 

While  a  great  deal  of  work  has  been  done  on  ultra-violet  light,  both 
in  Europe  and  in  America,  very  few  quantitative  investigations  have 
been  made.  Bell  and  Luckiesh  were  two  of  the  first  experimenters  to 
make  quantitative  investigations  along  these  lines.  Bell  (Electrical 
World,  April,  1912  and  Amer.  Acad.  Proc.  46,  April,  1911,  etc.),  by 
means  of  a  thermopile  and  sensitive  galvanometer,  has  obtained  valu- 
able data  on  the  ultra-violet  component  of  artificial  light  sources. 
Luckiesh  (Electrical  World,  June,  1912,  Illuminating  Eng.  Soc.,  April, 
1914,  Elect.  World,  May  24,  1913,  etc.)  used  a  photographic  method 
to  obtain  transmission  curves  of  various  kinds  of  glass. 

Ham,  Fehr  and  Bitner,  using  a  photographic  null  method  for  meas- 
uring absorption  in  the  ultra-violet,  published  their  results  (Journal 
of  the  Franklin  Institute,  Sept.,  1914)  upon  the  transmission  and  mini- 
mum lines  of  various  kinds  of  glass. 

In  1918  Coblentz  and  Emerson  (Technologic  Papers  of  the  Bureau 
of  Standards,  No.  93,  1918)  issued  a  paper  on  the  subject  of  Glasses 
for  Protecting  the  Eyes  from  Injurious  Radiations.  This  paper  deals 
quite  largely  with  the  visible  region  beyond  0.5/x  and  with  the  absorp- 
tion of  glasses  in  the  red  and  infra-red  regions.  Coblentz  and  Emer- 
son conclude :  ' '  For  protecting  the  eye  from  ultra-violet  light,  black, 
amber,  green,  greenish-yellow,  and  red  glasses  are  efficient.  Spectacles 
made  of  white  crown  glass  afford  some  protection  from  the  extreme 
ultra-violet  rays  which  come  from  mercury-in-quartz  lamps  and  from 
electric  arcs  between  iron,  copper  or  carbon.  The  vapors  from  these 
arcs  emit  but  little  infra-red  radiation  in  comparison  with  the  amount 
emitted  in  the  visible  and  in  the  ultra-violet." 

In  June  1919  Gibson  and  McNicholas  issued  a  paper  (Technologic 
Papers  of  the  Bureau  of  Standards,  No.  119,  1919)  on  the  Ultra-violet 
and  Visible  Transmission  of  Eye-Protective  Glasses,  in  which  they 
report  the  results  of  a  long  series  of  careful  spectrophotometric  obser- 


TRANSMISSION  OF  RADIANT  ENERGY  27 

vations  for  different  wavelengths  upon  various  eye-protective  media. 
A  logarithmic  relation  connects  the  transmittance  and  the  thickness 
of  glass,  thereby  enabling  a  direct  comparison  of  the  transmissions  and 
absorptions  of  various  kinds  of  glass  of  different  thicknesses.  A  con- 
siderable number  of  their  results  in  the  form  of  curves  appear  in  other 
portions  of  this  essay. 

Let  us  consider  some  of  these  experimental  methods  and  the  results 
of  various  investigators  somewhat  in  detail.  There  are  various  means 
and  methods  of  studying  the  transparency  of  media  for  the  ultra- 
violet. Photography  is,  without  doubt,  the  most  readily  applicable. 
The  radiometer,  thermopile  and  bolometer  could  be  used,  but  tempera- 
ture changes,  air  currents,  magnetic  disturbances  and  general  incon- 
veniences of  such  methods  bar  them  out  as  useful  in  such  investiga- 
tions as  are  now  being  discussed.  Photography  offers  several  distinct 
advantages:  among  these  may  be  mentioned:  (1)  Less  adjustment 
than  is  required  in  any  other  method,  (2)  extremely  faint  lines  can  be 
detected  and  measured  and  (3)  the  photographic  plate  gives  a  perma- 
nent record  of  the  test.  However,  be  it  said  that  when  the  transmission 
is  to  be  accurately  determined  the  photographic  method  is  a  very 
tedious  procedure.  The  photographic  action  is  determined  by  the 
density  of  the  plate.  In  plotting  the  density  of  the  plate  against  the 
logarithm  of  the  illustration  a  straight  line  relation  is  found  over  a 
certain  range  of  illumination.  By  no  means,  however,  is  the  density 
of  the  silver  deposit  proportional  to  the  logarithm  of  the  intensity  of 
radiation  throughout  any  extremely  wide  range  of  illumination.  Fur- 
thermore, rays  of  various  wavelengths  show  different  relations  between 
density  of  the  silver  deposit  and  the  illumination. 

Ham,  Fehr  and  Bitner  (Journal  of  the  Franklin  Inst.,  page  299, 
1914)  used  the  null  photographic  method  of  determining  transmis- 
sions. By  making  several  exposures  on  the  same  plates,  various  sources 
of  error  such  as  changes  in  temperature,  character  of  plate,  and  so 
forth,  could  be  eliminated  and  there  would  be  practically  no  errors 
introduced  due  to  the  emulsion  and  the  development.  By  making 
several  exposures  with  various  reduced  intensities  of  the  incident  beam 
a  very  close  match  could  be  obtained  between  two  adjacent  images  of 
the  same  spectral  line  and  a  fairly  close  estimate  of  the  absorption 
obtained  for  that  particular  wavelength.  For  example,  if  the  effect 
produced  by  the  original  beam  of  light  after  passing  through  the 
medium  were  the  same  as  the  effect  produced  by  the  beam  when  strik- 
ing the  plate  after  a  reduction-of  25  per  cent,  in  intensity,  the  absorp- 
tion of  the  medium  would  be  25  per  cent,  of  that  particular  wavelength. 
The  experimental  problem,  therefore,  consisted  of  two  parts:  (1)  the 


*  TRANSMISSION  OF  RADIANT  ENERGY 

determination  of  the  equality  of  the  densities  of  the  two  adjacent 
images  on  the  photographic  plate  and  (2)  the  reduction  of  the  incident 
beam  of  light  by  a  known  amount. 

Figure  13  (Ham,  Fehr  and  Bitner,  Journal  of  the  Franklin 
Institute,  Sept.,  1914)  shows  how  important  it  is  in  such  work  to  in- 
crease this  time  of  exposure  until  no  more  lines  appear  on  the  spectro- 
gram. "With  the  apparatus  used  it  was  found  that  an  exposure  of  120 
seconds  was  sufficiently  long  to  bring  out  the  line  of  minimum  wave- 


Fig.  13 — Transmission  of  clear  glass.      (After  Ham,  Fehr  and  Bitner,  Jour,  of 

Franklin  Inst.,  1914.) 
a.  Quartz,  10  seconds. 


: American'  clear  glass,     10  seconds. 

American'  clear  glass,     20  seconds. 

'American'  clear  glass,     30  seconds. 

American'  clear  glass,     60  seconds. 

American'  clear  glass,  120  seconds. 


length  in  all  the  glasses  tested,  whether  high  or  low  in  the  transmission 
of  visible  light. 

Figures  14,  15,  16  and  17,  taken  from  the  same  paper,  were  made 
with  exposures  of  120  seconds  each.  The  percentage  transmission  was 
obtained  by  the  use  of  a  1.25  watt  per  candle  tungsten  lamp  and  a 
flicker  photometer.  The  authors  say:  "In  looking  over  the  data  ob- 
tained in  regard  to  the  minimum  wavelength  transmitted,  some  very 
interesting  results  may  be  noted.  For  example,  the  faint  pink  of  No. 
4  transmits  as  much  ultra-violet  as  the  light  blue  of  No.  20.  Since 
both  glasses  have  about  equal  transmissions  for  visible  light,  it  was 
to  be  expected  that  the  one  nearer  the  red  end  of  the  spectrum  would 
cut  off  more  ultra-violet,  but  this  case  clearly  shows  that  no  dependence 
may  be  placed  on  the  color  of  the  glass.  Again,  Nos.  7  and  8,  of  very 
nearly  the  same  shade  of  yellow,  show  widely  different!  degrees  of 
transmission  of  ultra-violet,  the  'Noviol'  not  transmitting  even  all  of 


TRANSMISSION  OF  RADIANT  ENERGY  39 

the  visible  wavelengths  while  the  other  yellow  glass  transmits  as  far 
down  as  the  3150  t.  m.  line.  Euphos  glass  No.  11  cuts  off  the  ultra- 
violet very  sharply  at  4050  t.  m.  but  Nultra  glass  appears  to  be  some- 
what better  from  a  practical  standpoint  for  it  barely  transmits  the  3650 
line  (less  than  1  per  cent,  by  actual  test)  and  absorbs  only  15  per  cent, 
of  the  visible  light.  The  most  remarkable  glass  of  all  is  the  orange- 
yellow  of  No.  5  (figure  18)  which  appears  to  transmit  selectively  be- 


Fig.    14 — Minimum    lines    transmitted   by   various    glasses.      (After    Ham,    Fehr 
and  Bitner,  Jour.  FrankUn  Inst.,  1914.) 

Per  Cent. 
Transmission 

for  Tungsten  Minimum 

Light  at  Line 

No.     Absorbing  Medium                                                               1.25  w.p.c.  pp 

1.  Quartz    ....  .... 

2.  Very  deep  red  glass 4.1  ... 

3.  Red  glass 17.8 

4.  Faint  pink   50.1  313 

5.  Orange  yellow  38.3  334 

6.  Yellow 54.3  334 

7.  Yellow  ("Noviol") 78.0  546 

8.  Very  light  yellow 78.8  313 

tween  3340  and  4050  t.  m.,  although  its  greatest  transmission  is  at 
the  other  end  of  the  spectrum."  > 

One  criticism  of  this  work  of  Ham  and  his  colleagues  is  that  the 
mercury  arc  spectrum,  with  its  comparatively  few  lines,  was  used  as 
a  light  source.  Hence  the  minimum  transmission  may  not  be  the  mini- 
mum line  recorded.  The  remedy  lies  in  the  use  of  a  continuous 
spectrum,  or  as  nearly  continuous  as  is  obtainable.  A  method  of  under- 
water spark  has  been  already  referred  to  as  being  of  great  use  in  such 
work. 

Smith  and  Sheard   (Journal  of  (he  Optical  Society  of  America, 


50  TRANSMISSION  OF  RADIANT  ENERGY 

page  26,  1919)  made  use  of  a  condensed  spark  across  two  electrodes, 
one  made  of  iron,  the  other  of  an  alloy,  of  cadmium,  aluminium,  mag- 
nesium and  zinc.  Figure  18  gives  the  photographic  results  of  the  mini- 
mum transmissions  of  the  various  samples  of  glass  specified  at  the  side. 
A  Fery  quartz  spectrograph  was  used.  "The  times  of  exposure  were 
made  nearly  the  same  throughout  the  experiment.  The  intensity  of 
spark,  however,  fluctuated  so  much  that  it  is  not  possible  to  make  com- 
parisons concerning  the  amount  absorbed  by  the  different  thicknesses. 


9. 

10. 

11. 


15. 
16. 

Fig.  15. — Minimum  lines  transmitted  by  various  glasses.   (After  Ham,  Fehr  and 
Bitner,  Jowr.  Franklin  Inst.,  1914.) 

Per  Cent. 
Transmission 
for  Tungsten     Minimum 

Light  at  Line 

No.     Absorbing  Medium  1.25  w.p.c.  /*/* 

9.  Faint  yellow  ("Nultra")    85.0  365 

10.  Faint  yellow 84.3  313 

11.  Yellow  green  ("Euphos")    72.0  405 

12.  Yellow    green    71.6  365 

13.  Faint  yellow  green    82.5  302 

14.  Very  deep  green  2.6  405 

15.  Dark   green    5.0  365 

16.  Green     52.0  334 

*  *  *  It  was  the  purpose  of  this  part  of  the  experiment  to  show 
only  limits  to*  which  these  glasses  transmit  radiations  in  the  ultra- 
violet end  of  the  spectrum  for  fairly  long  exposures.  From  the  results 
obtained  it  is  seen  that  the  amethyst  and  the  blue  glasses  transmit 
farthest  into  the  ultra-violet.  They  seem  to  absorb  all  wavelengths 
beyond  3091  t.  m.  On  the  other  hand  the  deeper-colored  Noviol 
transmits  least  far  into  the  ultra-violet.  It  apparently  absorbs  every- 
thing beyond  about  5000  t.  m."  No  quantitative  measurements  are 
attached  to  these  spectrograms  but  they  do  point  out  the  fact  that 
ambers  of  various  kinds  (called  by  such  names  as  oliveye,  luxfel, 


TRANSMISSION  OF  RADIANT  ENERGY 


31 


Fig.  16. — Minimum  lines  transmitted  by  various  glasses.     (After  Ham,  Fehr  and 
Bitner,  Jour.  Franklin  Inst.,  1914.) 

Per  Cent. 
Transmission 
for  Tungsten     Minimum 


No.  Absorbing  Medium 

17.  Quartz    

18.  Dark  blue  violet  glass. . . . 

19.  Blue   - 

20.  Light  blue  ("Tungsten") 

21.  Eeddish  purple    

22.  Dark  flesh   color 


Light  at 
1.25  w.p.c. 

'2.5 

8.3 
46.1 
23.2 
44.3 


Line 


334 
334 
302 
334 
334 


Fig.    17. — Minimum    transmission    of   various   glasses.      (After   Ham,   Fehr   and 
Bitner,  Jour.  Franklin  Inst.,  1914.) 

Per  Cent. 
Transmission 
for  Tungsten     Minimum 


No.  Absorbing  Medium 

23.  Quartz    

24.  Light  flesh  color 

25.  Dark  gray 

26.  Medium  gray   

27.  Ground  glass    

28.  Clear  glass   ("American") 


Light  at 
1.25  w.p.c. 

73.8 

15.3 
36.8 
50.5 
90.2 


Line 
MM 

334 
334 
334 
365 
313 


32 


TRANSMISSION  OF  RADIANT  ENERGY 


Thickness 
of  Glass 

Condensed  Spark 

Euphos  5.0  mm 

Pf  und   2.6  mm 

Crookes  A   3.2  mm 

Crookes  B   2.8  mm 

Noviol  b 4.9  mm 

Noviol  a. 4.0  mm 

Smoke  No.  0 2.3  mm 

Smoke  No.  1 2.3  mm 

Smoke  No.  2 2.3  mm 

Luxf  el    2.4  mm 

Oliveye    2.5  mm 

Amethyst  No.  1 2.6  mm 

Amethyst  No.  2 4.8  mm 

Amethyst  No.  3 3.3  mm 

Resistal    3.4  mm 

Blue  No.  0 2.5mm 

Blue  No.  1 2.5  mm 

Blue  No.  2 2.5mm 

First  Amber 4.1  mm 

Light  Amber 2.5  mm. 

Medium  Amber    4.0  mm 

Dark  Amber   4.3  mm 

Dark  Amber  No.  2 ...  2.3  mm 

Light  Amber  No.  2 ...  2.4  mm 

Naetic  a 3.4  mm 

Naetic  21 3.3  mm 

Naetie  22 3.0  mm 

Nactie  23 3.2  mm 

Naetic  24  .  3.5mm 


Fig.  18.' — Transmission  of  various  ophthalmic  glasses  in  the  visible  and  ultra- 
violet. Spectrographic  record.  (Permission  of  the  Journal  of  the  Optical  Society 
of  America.) 

ambers  and  nactics,  etc.)  are  entirely  different  in  their  transmission 
limits  and  hence  in  the  amounts  of  various  wavelength  energy  which 
they  will  transmit.  The  family  of  spectrograms  labelled  ' '  Nactics ' '  at 
the  bottom  of  Figure  18  illustrates  this  point  very  nicely.  Neither, 
in  turn,  can  the  relative  limits  of  absorption  be  estimated  in  many 
cases  by  gradation  of  color  of  glass.  In  other  words,  coloring  chem- 
icals may  often  be  added  to  glass  without  appreciably  affecting  the 
limit  of  transmission,  as  instanced  by  Crookes  A  and  B  .shades,  in 


TRANSMISSION  OF  RADIANT  ENERGY 


33 


which  the  limit  of  transmission  in  the  ultra-violet  is  virtually  the  same. 
And  again,  various  samples  of  glass,  all  possessing  the  same  color  as 
judged  by  a  matching  of  samples  laid  on  a  sheet  of  white  paper,  may 
vary  considerably  in  their  limits  of  transmission. 

In  1914  Luckiesh  presented  a  paper  before  the  Illuminating  En- 
gineering Society  (Transactions  of  the  Illuminating  Eng.  Soc.,  Vol. 
9,  1914)  on  "Glasses  for  Protecting  the  Eyes."  Luckiesh  adopted  the 
procedure  of  using  the  light  from  a  quartz  mercury  arc  reflected  from 
a  magnesia  block.  A  series  of  exposures  of  equal  length  was  made 
but  with  different  known  illuminations  of  the  magnesia  surface.  Fol- 


Wavelength 

Fig.  19 — Transmission  of  glasses  in  the  region  3000  t.  m.  to  5000  t.  m.  (Courtesy 

of  M.  Luckiesh.) 

1.  Clear  lead  glass  4.  Light  amber  7.  Medium  amber 

2.  D  smoke  5.  7  smoke  8.  Euphos 

3.  Amethyst  6.  6  smoke  9.  Akopos 

lowing  these,  exposures  of  the  same  length  were  made  through  the 
media  to  be  examined  with  known  illumination.  The  photographs 
were  measured  for  density  on  a  Martin's  polarization  photometer  and 
curves  were  plotted  between  density  and  illumination  for  each  line. 
From  these  curves  the  corresponding  intensities  of  illumination  (i.  e., 
transmission)  were  read  off  for  each  line  of  each  negative  exposed 
through  the  specimens.  By  taking  into  account  the  relative  illumina- 
tions of  the  magnesia  block  the  transmission  at  each  wavelength  was 
readily  obtained.  Figure  19  shows  the  transmission  curves  of  various 
glasses  in  the  region  of  0.3/u  to  0.5/i  (3000  to  5000  t.  m.).  The  curves 
are  numbered  and  the  glasses  giving  these  transmissions  are  tabulated 
above  the  curves.  Luckiesh  says:  "It  will  be  noted  that  the  trans- 


34 


TRANSMISSION  OB!  RADIANT  ENERGY 


Visible  Ultra-violet 


Group  A. 
1.  Iron  arc  (bare) 


2.  Iron   car- 

bon   arc 

3.  Iron  car- 

bon   arc 

4.  Iron   car- 

bon   arc 

5.  Iron    car 

bon    are  j 

6.  Carbon  arc 


Iron  above. . 
Iron  positive 
Iron  above. . 
Iron  negative 
Iron  below. . 
Iron  positive 
Iron  below. . 
Iron  negative 


Group  B. 

7.  Iron  arc  (bare)   

8.  Oxy-acetylene  welders,  K. . 

9.  Smoke,  No.  8  shade. 

10.  Akopos    

11.  Oxy-acetylene  welders,  K. . 

12.  Smoke,   No.   8   shade 

13.  Akopos    

Group  C. 

14.  Quartz  mercury  arc 

15.  Oxy-acetylene  welders,  K. . 

16.  Smoke,  No.  8  shade 

17.  Akopos    

18.  Oxy-acetylene  welders,  K.. 

19.  Smoke,   No.   8   shade 

20.  Akopos    

Group  D. 

21.  Iron  are  (bare)   

22.  Quartz  mercury  are 

23.  Fieuzal,  light 

24.  Clear  glass  

25.  Cobalt  blue,  dense 

26.  Celluloid    

27.  Signal  green,  medium 


Exposure 

(Seed's  23) 

see. 


V* 


y* 


3 
3 
3 

10 
10 
10 


Trans- 
mission 

coefficient 
of  speci- 
men for 
tungsten 
light  at 

1.25  w.p.c. 


3 

0.002 

3 

0.04 

3 

0.50 

10 

0.002 

10 

0.04 

10 

0.50 

0.002 

0.04 

0.50 

0.002 

0.04 

0.50 


0.86 
0.92 
0.02 
0.60 
0.001 


Fig.  20 — Spectrograms  of  the  transmission  of  various  glasses  in  the  ultraviolet. 
(Courtesy  of  M.  Luckiesh.) 


TRANSMISSION  OF  RADIANT  ENERGY 


35 


Visible  Ultra-violet 


Group  E. 

28.  Quartz  mercury  arc... 

29.  Clear  glass,  ^le"  thick. 

30.  Amber,  medium,   X... 

31.  Kosma  

32.  Electric  smoke 

33.  Smoke  X   


Group  F. 

34.  Quartz  mercury  arc. 

35.  Amber,   medium,  X. 

36.  Kosma   

37.  Smoke  X 

38.  Euphos,  %4"  thick  . 

39.  Thermoscopic   


Group  G. 

40.  Quartz  mercury  arc   

41.  Distilled  water   (1  cm.) . . . 

42.  Clear  glass,  %6"  thick 

43.  Smoke  A 

44.  Smoke  C   

45.  Smoke  A  +  C <-... 

Group  H. 

46.  Quartz  mercury  are   

47.  Euphos,  %4"  thick 

48.  Amber,   light  shade 

49.  Amber,  medium  shade 

50.  Amber,  medium,  X 

51.  Smoke  X    . 


Exposure 

(Lantern 

slide 

plate) 

sec. 


¥2 
4 
1 

480 
3 


15 


y2 


y2 


y2 


i 

3 
6 

20 
15 


Trans- 
mission 

coefficient 
of  speci- 
men for 

tungsten 
light  at 

1.25  w.p.c. 


0.92 
0.50 
0.83 
0+ 
0.015 


0.50 

0.83 

0.015 

0.81 

0.86 


0.93 
0.92 
0.36 
0.20 

0.07 


0.90 
0.67 
0.45 
0.50 
0.015 


Fig.  21 — Spectrograms  of  the  transmission  of  various  glasses  in  the  ultraviolet. 
(Courtesy  of  M.  Luckiesh.) 


36 


TRANSMISSION  OF  RADIANT  ENERGY 


Visible  Ultra-violet 


52. 
53. 
54. 
55. 
56. 
57. 
58. 

59. 
60. 
61. 
62. 
63. 
64. 
65. 

66. 
67. 
68. 
69. 
70. 
71. 
72. 


Group  I. 
Iron  are    (bare) 
Clear  glass,  y^"  thick 
Cobalt  blue,  dense . . . 

Canary,  light    

Signal  green,  medium 
Signal  blue,  medium 
Green,   dense    

Group  J. 
Iron  arc  (bare)   .... 

Clear  glass   

Cobalt  blue,  dense. . 
Gelatine  film,  purple 
Gelatine  film,,  blue . . 
Gelatine  film,  green 
Gelatine  film,  orange 

Group  K. 
Quartz  mercury  are 

Smoke  CK    

Smoke-  CK    

Smoke  CK    

Smoke  DK   . 


Smoke  DK 
Smoke  DK 


Exposure 

(Seed's  23) 

sec. 


Trans- 
mission 

coefficient 
of  speci- 
men for 
tungsten 
light  at 

1.25  w.p.c. 


% 

0.92 

2 

0.02 

2 

0.82 

8 

0.001 

4 

0.11 

8 

0.20 

2 

0.92 

2 

0.02 

2 

0.17 

2 

0.43 

2 

0.67 

2 

0.53 

i, 

Lantern  Slide 

f 

•7 
2 

2 

0.001 

10 

0.001 

20 

0.001 

1 

0.15 

2 

0.15 

5 

0.15 

Pig.  22 — Spectrograms  of  the  transmission  of  various  glasses  in  the  ultraviolet. 
(Courtesy  of  M.  Luckiesh.) 


TRANSMISSION  OF  RADIANT  ENERGY 


40      45        50       55      60        65       70       75     40 


50      55       60       65       70      75 


Fig.  23 — Charaeteristie  transmission  curves   for  colored  glasses.      (From,  Gage: 
Trans.  III.  Eng.  Soc.,  Vol.  XI,  1916.) 


JVKXH  BLUE 

(GREEN  [YELLOW 

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Fig.   24 — Characteristic  transmission  curves  for  colored  glasses.      (After  Gage: 
Trans.  Ill  Eng.  Soc.,  Vol.  XI,  1916.) 


38 


TRANSMISSION  OF  RADIANT  ENERGY 


parency  of  clear  lead  glass  remains  unchanged  to  rays  as  short  as 
0.35/x.  The  smoke  glasses  are  representative  of  many  examined.  They 
show  little  tendency  to  selectively  absorb  ultra-violet  rays  and  differ 
considerably  in  their  characteristics.  These  glasses  cannot  conscien- 
tiously be  recommended  with  safety  for  the  protection  of  the  eyes 
against  excessive  ultra-violet  radiation.  The  amethyst  glass  absorbs 
more  ultra-violet  than  clear  glass  yet  is  transparent  far  into  the  ultra- 


Fig.  1.    Bed 


Fig.  2.     Yellow 


Fig.  3.     Green 


Fig.  4.     Lunar  white 


Fig.  5.     Blue 


Fig.  25 — Spectrograms  of  colored  glasses  used  in  railway  signals.     (After  Gage: 
Trans.  III.  Eng.  Soc.,  Vol.  XI,  1916.) 

violet  region.  The  amber  glasses  quite  satisfactorily  absorb  ultra- 
violet rays  but  give  rise  to  some  objection  from  the  standpoint  of 
color.  Several  deep-red  glasses  were  examined  and  all  found  to  be 
opaque  to  ultra-violet  rays  but  on  account  of  the  strong  color  should 
not  be  recommended.  *  *  *  The  transmission  of  Euphos  glass 
decreases  considerably  in  the  ultra-violet  but  shows  a  tendency  to  in- 
crease in  transparency  in  the  region  3200  t.  m.  This  transparency 
to  short  wave  ultra-violet  rays  becomes  quite  marked  in  less  dense 
specimens." 

Figures  20,  21  and  22  give  a  number  of  spectrograms  published  by 
Luckiesh  illustrating  the  transparency  of  various  media  to  rays  of 


TRANSMISSION  OF  RADIANT  ENERGY  39 

different  wavelength.  The  division  line  between  the  visible  and  ultra- 
violet is  set  at  about  4000  to  3900  t.  m.  The  seconds'  exposure  and 
the  transmission  coefficient  for  the  total  visible  light  from  a  tungsten 
lamp  are  indicated.  "In  group  I  and  J,"  writes  Luckiesh,  "are  the 


Fig.  1.     Cobalt 


Fig.  2.     Didymium 


Fig.  3.     Uranium 


Fig.  4.     Chrome  green 


Fig.  5.     Blue  green 


Fig.  6.     Blue  green  plug  noglare 


Fig.  7.     Clear 


Fig.  26 — Colored  glasses  exhibiting  band  spectra   (Nos.  1  to  3).     Green  glasses 
(Nos.  4  to  6).     (After  Gage:  Trcms.  III.  Eng.  Soo.,  1916.) 

spectrograms  of  various  common  glasses  which  are  often  used  in  the 
industries.  It  is  seen  that  the  cobalt-blue  glass  is  more  transparent 
to  ultra-violet  radiation  than  is  a  clear  glass  of  the  same  thickness. 
The  clear  glass  used  was  a  lantern-slide  cover  glass.  This  difference 
is  best  shown  in  60  and  61  where  the  exposures  were  equal.  It  is  signi- 


40  TRANSMISSION  OF  RADIANT  ENERGY 

ficant  to  note  that  the  clear  glass  is  approximately  46  times  the  more 
transparent  to  visible  light  than  the  cobalt-blue  glass." 

Figures  23-26  are  taken  from  an  article  by  Dr.  H.  P.  Gage  on 
"Colored  Glass  in  Illuminating  Engineering,"  (Trans.  III.  Eng.  Soc., 
Vol.  XI,  1916).  The  first  two  of  these  diagrams  give  some  charac- 
teristic curves  of  transmission  in  the  visible  regions  by  various  colored 
glasses.  Figure  25  gives  reproduced  spectrograms  of  the  colored 
glasses  commonly  used  in  railway  signals.  Figure  26  is  of  interest 
in  that  it  shows  the  effects  of  the  addition  of  certain  ingredients  to 
white  glass  upon  the  transmissive  properties  of  the  product. 

The  transmission  curves  of  a  considerable  number  of  neutral  and 
colored  glasses  have  been  determined  in  the  laboratories  of  the  Ameri- 
can Optical  Company  and  published  in  a  brochure  entitled  "The 
Ophthalmic  Use  of  Crookes  Lenses."  These  curves  for  clear  glass, 
Crookes  A  and  B,  a  couple  of  ambers,  amethyst,  smoke,  etc.,  together 
with  the  data  giving  the  approximate  absorptions  are  shown  in  Figures 
27-35,  inclusive.  It  will  be  noted  that  the  transmission  in  the  visible 
spectrum  for  white  glass  is  practically  92  per  cent.,  the  reflection  from 
the  surfaces  amounting  to  about  8  per  cent.  The  ultra-violet  and  near 
violet  transmission  is  shown  by  shading  in  two  different  manners,  that 
lying  close  to  the  violet  (3900  to  3700  t.  m.  roughly),  and  that  below 
3700  t.  m.  The  transmission  for  white  glass  becomes  zero  at  about 
the  2800  t.  m.  point.  It  will  also  be  noticed  that  the  transmission  in 
the  ultra-violet  is  considerably  greater  than  that  of  any  of  the  other 
glasses  shown.  The  Wellsworth  Crookes  A — after  the  formula  of  Sir 
William  Crookes — is  practically  a  colorless  glass  and  yet  the  absorp- 
tion of  the  ultra-violet  in  comparison  to  white  glass  is  marked,  absorb- 
ing as  it  does  the  ultra-violet  completely  below  3600  t.  m.  Another 
characteristic  of  both  the  Crookes  A  and  B  shades  is  the  appreciable 
absorption  of  a  selective  character  in  the  yellow  region  and  just  above 
the  point  generally  specified  as  being  the  maximum  of  the  sensibility 
curve  of  the  average  eye  (5600  t.  m.)  under  fairly  high  illuminations. 
The  reader  will  likewise  be  interested  in  comparing  these  curves  for 
"Wellsworth  Crookes  lenses  with  determinations  made  at  the  Bureau 
of  Standards  by  Gibson  and  McNicholas  (Figure  36).  And  again,  a 
comparison  of  Wellsworth  Crookes  A  and  B,  either  in  the  diagrams 
accompanying  this  discussion  or  in  those  given  by  Gibson  and  Mc- 
Nicholas, show  that  the  limits  of  absorptipn  in  the  ultra-violet  of  the 
two  shades  is  the  same  and  that  the  transmission  curves  are  practically 
one  and  the  same  beyond  3700  t.  m.  Figure  36,  by  Gibson  and  Mc- 
Nicholas, gives  the  transmission  curves  of  white  glass,  Crookes  A 
Wellsworth,  Crookes  B  Wellsworth  side  by  side  upon  the  same  plat 


TRANSMISSION  OF  RADIANT  ENERGY 


WHITE  GLASS 


Fig.  27.     White  Glass. 

Absorption  of  white  optical  glass 
used  for  spectacles  for  a  thickness  of 
2  mm.  is  under  1/10%,  throughout  the 
visible  spectrum.  The  transmission  of 
a  piano  white  lens  is  91.8%.  The  re- 
flection from  the  two  surfaces  is  8.2%. 


Pig.    28.      Wellsworth    Crookea    "A" 

The  curve  shows  the  percentage 
transmission  of  Crookes'  "A"  for 
various  colors  or  wave-lengths  of  light 
(thickness  2  mm.). 

The  percentage  of  absorption  for 
these  different  wave-lengths  is  as  fol- 
lows : 

Limit-Red         1%  Blue       7% 

Red         1  Violet    8 

Yellow  15  Limit-Violet  15 
Green     4 


Fig.    29.     Amber   "B" — Selected 

Quality 

The  curve  shows  the  percentage 
transmission  of  Amber  (dark  shade) 
for  various  colors  and  wave-lengths  of 
light  (thickness  2  mm.).  The  per- 
centage of  absorption  for  these  dif- 
ferent wave-lengths  is  as  follows : 
Limit-Red  20%  Blue  65% 

Red       25  Violet  90 

Yellow  30  Limit-Violet  95 

Green  45 


Fig.   30.     Euphos   "A" 

The  curve  shows  the  percentage 
transmission  of  Euphos  for  various 
colors  and  wave-lengths  of  light. 

The  percentage  of  absorption  for 
these  different  wave-lengths  is  as 
follows : 

Limit-Red       20%  Blue     45% 

Red       25  Violet  85 

Yellow  15  Limit  Violet  99 

Green  15 


Fig.  31.     Amethyst—  Medium  Shade. 

The  curve  shows  the  percentage 
transmission  of  Amethyst  (dark 
shade)  for  various  colors  and  wave- 
lengths of  light  (thickness  2  mm.). 
The  percentage  of  absorption  for  these 
different  wave-lengths  is  as  follows  : 
Limit-Red  10%  Blue  60% 

Red       15  Violet  10 

Yellow  20  Limit-Violet    8 

Green  28 


Figs.  27-31  —  Percentage  transmission  of  light  by  ophthalmic  glasses.     (From  the 

American  Optical  Co.) 


42 


TRANSMISSION  OF  RADIANT  ENERGY 


and  this  makes  comparison  easy.  Another  interesting  comparison  is 
that  between  Crookes  B  and  Smoke  B  (vide  Figures  32  and  35).  In 
many  respects  these  glasses  are  similar  in  their  absorptions  but  there 
is  one  marked  and  noteworthy  difference :  the  absorption  of  the  ultra- 


WCLLSWOFtTH  CROOKES   B 


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Y 

n 

-  7SX- 

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e 

E 

E 

L 

D 

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N 

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300        KO         40O        150         500       550          fcOO       «SO  mu."     ~ 

4  I  I  I  I        '    I  I  \       r 


.,      SMOKE    B 


Wellsworth    Crookes    "B" 
shows     the    percentage 


Fig.    32. 

The     curve 

transmission  of  Crookes'  "  "B"  for 
various  colors  or  wave-lengths  of  light 
(thickness  2  mm.).  The  percentage  of 
absorption  for  these  different  wave- 
lengths is  as  follows : 
Limit-Red  15%  Blue  50% 

Red       35  Violet  30 

Yellow  40  Limit-Violet  20 

Green  40 

Fig.  33.     Amber  "A" — Selected 

Quality 

The  curve  shows  the  percentage 
transmission  of  Amber  (light  shade) 
for  various  colors  and  wave-lengths  of 
light  (thickness  2  mm.). 

The  percentage  of  absorption  for 
these  different  wave-lengths  is  as  fol- 
lows : 

Limit-Red         5%  Blue     25% 

Red         8  Violet  30 

Yellow  10  Limit-Violet  40 

Green  15 

Fig.  34.     Noviol  "O" 
The     curve     shows     the     percentage 
transmission  of  Noviol  "O"  for  various 
colors  or  wave-lengths  of  light   (thick- 
ness 2  mm.). 

The  percentage  of  absorption  for 
these  different  wave-lengths  is  as  fol- 
lows : 

Limit-Red          1%  Blue     15% 

Red         2  Violet  50 

Yellow    4  Limit- Violet  75 

Green     4 

Fig.  35.     Smoke  "B" 
The     curve     shows     the     percentage 
transmission       of       Smoke       (medium 
shade)       for  various  colors  and   wave- 
lengths  of  light    (thickness  2  mm.). 

The  percentage  •  of  absorption  for 
these  different  wave-lengths  is  as  fol- 
lows : 

Limit-Red          5%  Blue     55% 

Red       10  Violet  40 

Yellow  50  Limit-Violet  25 

Green  50 


Figs.  32-35 — Percentage  transmission  of  light  by  ophthalmic  glasses.     (From  the 

American  Optical  Co.) 


violet  by  Crookes  B  is  complete  at  slightly  under  3600  t.  m.,  while  for 
the  Smoke  B  the  absorption  is  not  complete  until  about  3200  t.  m.  is 
reached.  There  are  many  reasons  for  believing  that  the  deleterious  or, 
to  say  the  least,  non-desirable  effects  of  the  ultra-violet  rays  under  ordi- 
nary, workaday  conditions  lie  in  the  region  just  below  3600  t.  m. 
Since  smoke  glass  transmits  these  shorter  ultra-violet  rays  it  would 
appear  fairly  conclusive  that  the  value  of  the  use  of  Crookes,  Smoke 


TRANSMISSION  OF  RADIANT  ENERGY 


43 


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Fig.  36— A=" Crookes  A,  Wellsworth,"  2.05  mm;  B="Crookes  B,  Wells- 
worth,"  2.16  mm;  A.  O.  Co.  d="91  B,"  1.97  mm;  Corning.  (After  Gibson 
and  McNicholas.  Permission  of  Bureau  of  Standards.) 


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VLTM-VKX.CT  VIOLtT       SLUC  (RUN     ItllOWORANSC  KCO  WR 

WVC-LCN6TH,  MCTCRS'IO'9 

Fig.  37 — A— "Smoke  A,"  2.10  mm;  B=" Smoke  B,"  2.14  mm;  C=" Smoke 
C,"  2.13  mm;  I)—" Smoke  D,"  2.03  mm;  A.  O.  Co.  (After  Gibson  and  Mc- 
Nicholas. permission  of  Bureau  of  Standards.) 


44 


TRANSMISSION  OF  RADIANT  ENERGY 


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Fig.  38—  A="Luxfel,"  2.00  mm;  B="Lab.  No.  57,"  1.90  mm;  C=:"Lab. 
No.  58,"  2.02  mm;  A.  O.  Co.  (After  Gibson  and  McNicholas.  Permission  of 
Bureau  of  Standards.) 


ItO 


200.    &0     ZiO     320    3tO    400     440    480    5ZO    5M     tOO     bH 

1/lTKA-mx.CT  Y10LCT    BLUE  GRC  £/V  YCU.OW  OHM1SC 


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Fig.  39—  AA="Noviol  AA,"  1.88  mm;  O="Noviol  O,"  2.03  mm;  A—  "Noviol 
A,"  2.06  mm;  B="Noviol  B,"  2.10  mm;  C="Noviol  C,"  2.10  mm;  A.  O.  Co. 
(Permission  of  Bureau  of  Standards.) 


TRANSMISSION  OF  RADIANT  ENERGY 


45 


or  similar  glasses  lies  in  the  reduction  of  the  total  quantity  of  energy 
entering  the  eye.  A  comparison  of  the  data  of  the  curves  given  in 
Figures  36  and  37  will  emphasize  these  points  of  similarity  and  dis- 
similarity. Amber  A  (Fig.  33),  Amber  B  (Fig.  29)  and  Noviol  O 
(Fig.  34)  may  properly  be  grouped  in  a  family  for  the  purposes  of 
discussion.  All  of  these  (and  other  glasses  such  as  Nactic,  Luxfel, 
Oliveye,  etc.)  possess  in  general  a  yellowish  or  yellowish-green  hue. 
The  Noviol  0,  of  all  those  specified,  has  the  least  effect  upon  the  visible 


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Fig.  40—  A="  Amber  A,"  2.13  mm;  B—  "Amber  B,"  2.13  mm;  A.  O.  Co.  0= 
"Amber,  light,"  1.93  mm;  D—  "Amber,  dark,"  1.85  mm;  W.  &  O.  (Permission 
of  Bureau  of  Standards.) 

spectrum,  since  it  transmits  about  85  per  cent,  of  all  wavelengths  from 
the  red  at  7200  t.  m.  down  to  the  green-blue  at  5000  t.  m.  The  amber 
A  (selected  quality,  Fig.  33)  transmits  considerably  more  ultra-violet 
than  does  the  Noviol.  In  turn,  however,  the  amber  B  absorbs  all  the 
ultra-violet  beyond  "3800-3900  t.  m.  and  has  much  more  marked  ab- 
sorptive effects  in  the  violet,  blue  and  green  than  either  Noviol  or 
the  lighter  ambers.  The  transmission  curves  for  Euphos,  Fieuzal, 
Chlorophil,  Hallauer,  Akopos,  Saniweld,  and  special  glasses  such  as 
"392  F,"  "124  J.  A,"  etc.,  and  the  Pfund  gold  film  between  plates 
of  Crookes  glass  are  given  in  Figures  41  to  44. 

Eeference  has  already  been  made  to  the  work  of  Gibson  and  Mc- 
Nicholas  6n  the  Ultra-violet  and  Visible'  Transmission  of  Eye-Pro- 


46 


TRANSMISSION  OF  RADIANT  ENERGY 


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Fig.  41—  A="Euphos,"  1.95  mm;  B="Lab.  No.  61,"  2.13  mm;  A.  O.  Co. 
C=glass  labeled  "Fieuzal,"  bought  .  in  store.  (Permission  of  Bureau  of 
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Fig.  42— A^'Fieuzal  A,"  2.13  mm;   B— "Fieuzal  B,"  2.13  mm;   A.  O.  Co. 
3=" Fieuzal, "  1.98  mm,  W.  &  O.   (Permission  of  Bureau  of  Standards.) 


TRANSMISSION  OF  RADIANT  ENERGY 


47 


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Fig.  43—  A="Fieuzal,"  1.98  mm;  B="  Chlorophile,  "  1.98  mm;  C="Hal- 
lauer,"  1.90  mm;  W.  &  O.  D="Akopos,"  2.17  mm;  E="  Saniweld,  light," 
1.82  mm;  F="  Saniweld,  dark,"  2.12  mm  (see  Fig.  21);  King.  (Permission 
of  Bureau  of  Standards.) 


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WMHING7H.  flCJTKS  XICT* 

Fig.  44— A="392  F,"  1.90  mm;  B="124  JA,"  2.02  mm;  C="124  IP,"  2.00 
mm  (see  Fig.  21);  Corning.  D="Lab.  No.  59,"  2.13  mm;  E="Pfund,"  gold 
film  between  plates  of  "Crookes"  glass,  total  2.89  mm;  A.  O.  Co.  (Permission 
of  Bureau  of  Standards.) 


5  TRANSMISSION  OF  RADIANT  ENERGY 

tective  Glasses.  This  appeared  as  one  of  the  Technologic  Papers  of 
the  Bureau  of  Standards  in  1919,  (No.  119).  Without  doubt  it  is  the 
most  exhaustive  study  of  the  subject  yet  made.  For  details  of  the 
experimental  procedure  and  for  the  methods  they  devised  of  com- 
puting the  transmission  for  different  thicknesses  of  glass,  the  reader 
is  referred  to  the  original  paper.  Figures  36  to  47  inclusive  are  taken 
from  the  paper. by  Gibson  and  McNicholas  and  are  self-explanatory. 
In  commenting  upon  the  results  of  their  investigations  these  men 


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ViTffrHOLCT  VIOLET    KMC  GRKH    YELLOW  OKAN6C         RED 


a? 


JtO  IX 


Fig.  45—  A="  Amethyst    A,"     2.08     mm;     B="  Amethyst    B,"     2.05    mm; 
C="  Amethyst  C,"  2.04  mm;  A.  O.  Co.     (Permission  of  Bureau  of  Standards.) 

write  :  "Of  the  specimens  studied,  the  five  kinds  which  are  most  effi- 
cient as  protection  against  the  ultra-violet,  while  being  at  the  same 
time  nearly  colorless  in  the  thicknesses  examined,  are  'Crookes  A,' 
Corning  '91  B,'  A.  O.  Co.  Lab.  'No.  57,'  A.  0.  Co.  Lab.  'No.  58,'  and 
'Noviol  0.'  Of  these,  'Noviol  O'  and  A.  0.  Co.  'Lab.  No.  58'  are  the 
best,  but  are  not  so  truly  colorless  as  the  other  three.  Of  the  slightly 
colored  glasses,  by  far  the  best  seem  to  be  'Noviol  A'  and  'Noviol 
A!  '  as  they  absorb  completely  below  410  m/*.  while  transmitting  about 
87  per  cent,  of  the  incident  light.  It  is  not  thought  that  the  slight 
color  would  be  at  all  objectionable  for  ordinary  use.  A  combina- 
tion of  'Noviol  A'  and  Corning  '124JA'  is  very  efficient  for  eye 
protection,  as  it  absorbs  all  the  ultra-violet  and  most  of  the  infra- 


TRANSMISSION  OF  RADIANT  ENERGY 


49 


red,  and  still  has  high  visible  transmission.  The  color  is  a  very  light 
green  and  the  colors  of  objects  viewed  through  it  are  distorted  prac- 
tically none  at  all.  A  gold  film  on  'Noviol  A'  glass  would  also  be 
very  efficient,  though  transmitting  less  of  the  visible  than  the  com- 
bination just  mentioned.  The  yellow  and  yellow-green  glasses  of  a 
deeper  shade  are  usually  good  protection  against  the  ultra-violet. 
The  green  and  blue-green  glasses  of  Fig.  44  are  used  primarily  to 
protect  the  eye  from  the  infra-red.  The  'Pfund'  specimen  is  a  gold 


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Fig.  46—  A=<  '  Blue  A,  '  '  2.10  mm  ;  B=<  «  Blue  B,  '  '  2.04  mm  ;  C='  '  Blue  C,  '  '  2.05 
mm;  D:="Blue  D,  "  2.11  mm;  A.  6.  Co.  (Permission  of  Bureau  of  Standards.) 

film  between  two  pieces  of  what  .seems  to  be  'Crookes'  glass. 
'Smoke/  amethyst  and  blue  or  purple  glasses  are  liable  to  be  little 
better  than  clear  glass  as  a  protection  against  the  ultra-violet.  Of 
the  welding  glasses,  yellow  seems  to  be  the  safest,  as  the  green  or 
neutral  shades  are  liable  to  have  transmission  bands  centering  near 
395  m/i,  which  may  extend  to  a  considerable  distance  into  the  ultra- 
violet. 

Gibson  and  McNicholas  also  investigated  the  transmissions  of  a  few 
glasses  which  may  be  classed  as  welding  glasses.  These  glasses  are  for 
use  under  high  powered  arcs  and  chiefly  in  industries  in  which  weld- 
ing enters.  Figures  48,  49  and  50  give  the  graphical  results  in  the 
ultra-violet  and  visible  of  several  kinds  of  special  welding  glasses. 


50 


TRANSMISSION  OF  RADIANT  ENERGY 


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Fig.  47—  AAnz"  Cobalt  Blue  AA,"  2.75  mm;  B="  Cobalt  Blue  B,"  1.85  mm; 
D="  Cobalt  Blue  D,"  1.86  mm;  "Cobalt  Blue  A,"  3.20  mm  nearly  same  as  curve 
AA;  "Cobalt  Blue  C,  "  1.46  mm  nearly  same  as  curve  B;  "Chromatic  Test,"  2.36 
mm  similar  to  curve  B,  but  slightly  lower  in  value;  A.  O.  Co.  (Permission  of 
Bureau  of  Standards.) 


430.8 


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Fig.  48—  A="  Welders  Smoke  Dark,"  1.42  mm;  B="  Special  Welders 
Light,"  1.68  mm  (see  Fig.  21);  C="  Special  Welders  Dark,"  2.54  mm  (see 
Fig.  21);  A.  O.  Co.  (Permission  of  Bureau  of  Standards.) 


TRANSMISSION  OF  RADIANT  ENERGY 


51 


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Fig.  49 — A— "Welding  glass  6,"  1.97  mm;  B=z" Welding  glass  2,"  2.32  mm; 
C.  E.  S.  Co.  C="Noviweld  4,"  1.89  mm;  D="Noviweld  5,"  2.16  mm;  E=: 
"Noviweld  6,"  2.20  mm;  "Noviweld  7,"  1.90  mm  1.02%  at  578;  "Noviweld  8," 
2.01  mm  0.517%  at  578;  A.  O.  Co.  (Permission  of  Bureau  of  Standards.) 


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Fig.  50 — Note  scale  of  ordinates.  A=" Special  Noviweld  No.  8,"  1.77  mm; 
Hardy.  B— "391  DD,"  1.90  mm;  Corning.  (Permission  of  Bureau  of  Stand- 
ards.) . 


52  TRANSMISSION  OF  RADIANT  ENERGY 

The  extremely  small  percentage  (about  0.03  per  cent.)  of  transmis- 
sion in  the  yellow-green  region  for  the  special  Noviwelds  shown  in 
Figure  50  is  worthy  of  notice. 

One  criticism  which  it  seems  to  the  writer  may  be  passed  upon  the 
work  of  Gibson  and  McNicholas  is  that,  in  the  majority  of  their  tests, 
transmission  measures  in  the  ultra-violet  were  not  carried  further 
than  2  per  cent.  With  low  incident  energy  or  brief  times  of  exposure 
this  small  amount  of  transmission  in  the  extreme  ultra-violet  ob- 
tained with  glasses,  etc.,  might  be  neglected,  but  this  low  percentage 
transmission  may  still  be  extremely  important  under  conditions  im- 
posed by  welding  operations  and  so  forth. 


Fig.   51 — Effect  of  thickness  on  transmission  of  Wellsworth  Crookes  glass. 
(Permission  of  the  American  Optical  Company.) 

In  all  of  these  later  experiments  upon  the  transmission  of  glass 
due  care  has  been  taken  to  have  all  the  results  reduced  to  a  uniform 
basis;  i.  e.,  the  element  of  variation  of  thickness  of  sample  of  glass 
has  been  eliminated.  It  is  of  interest,  therefore,  to  see  what  effects 
thickness  has  upon  the  transmission  of  glass  having  thicknesses  found 
in  ophthalmic  lenses.  An  article  by  Sheard  on  the  Effect  of  the 
Thickness  of  Glass  upon  the  Transmission  of  Various  Parts  of  the 
Spectrum  (Wellsworth,  page  140,  1919)  gives  the  results  upon  Crookes 
A,  Noviol  0,  Smoke  B  and  Fieuzal  A.  Every  lens  other  than  a  piano 
has  a  thickness  varying  from  the  center  to  the  edge.  In  the  case  of 
convex  lenses,  the  thickness  increases  toward  the  position  of  the  optical 
center:  in  concave  lenses  just  the  reverse  is  true.  Hence,  a  high- 
powered  plus  lens  may  be  several  millimeters  in  thickness  at  the  optical 
center,  while  a  concave  lens  may  be  almost  as  thin  as  a  sheet  of  paper 
at  the  center  point.  It  necessarily  follows,  therefore,  that  the  tint 
of  a  lens  cannot  be  preserved  uniformly  over  the  surface  of  lenses 
having  appreciable  power.  An  amber  lens,  for  example,  becomes 


TRANSMISSION  OF  RADIANT  ENERGY 


lighter  and  the  color  fades  out  at  the  thinner  portions  of  the  finished 
lens,  although  the  original  glass  block  from  which  the  lens  was  man- 
ufactured was  of  uniform  color  or  tint  throughout.  As  a  result 
therefore,  the  color  and  the  percentage  of  transmitted  light  of  va- 
rious wavelengths  vary.  .There  is,  possibly,  one  method  which  would 
annul  such  effects  and  that  is  the  scheme  of  applying  the  tint,  some- 
what after  the  manner  of  a  coat  of  paint,  to  the  surfaces  of  the  lenses 
after  they  are  finished.  But  no  satisfactory  device  has  yet  been 
discovered  which  will  give  the  effect  and  the  durability  needed.  And 
it  is  to  be  seriously  doubted  whether  this  process  of  coating(  lenses 
with  desired  colors,  even  if  discovered,  would  obviate  the  changes 


300  3SO  400  ISO  500  650  «X> 


65O  700 


Fig.  52 — Effect  of  thickness  on  the  transmission  of  Noviol  O.     (From  the  American 

Optical  Company.) 

in  shade  or  tint,  since  the  same  fundamental  problem  of  absorption 
of  glass  as  dependent  upon  its  thickness  would  again  enter. 

The  four  sets  of  curves  given  in  Figures  51-54  were  determined 
in  the  Research  Laboratories  of  the  American  Optical  Com- 
pany. They  are  given  as  typical  curves  illustrative  of  the  effects  of 
glass  upon  the  transmission  of  spectral  energy  from  the  red  down 
through  and  including  the  ultra-violet.  In  each  of  these  diagrams 
the  upper  curves  represent  the  transmission  through  one  centimeter 
of  glass  and  the  lower  curves  give  the  results  when  the  thickness  of 
glass  is  of  the  order  of  three  millimeters. 

The  curves  show  that  a  change  of  thickness  from  1mm.  to  3  mms. 
has  but  little  effect  upon  the  transmission  of  the  visual  and  the  ultra- 
violet radiations  in  the  case  of  Crookes  A  and  Noviol  0.  The  greatest 
effect  of  thickness  in  Crookes  A  lies  in  the  yellow-green  region  (5500 
to  6000  Angstroms)  where  the  absorption  in  the  two  characteristic 
bands  is  increased  almost  25  per  cent.  The  average  effect  of  tripling 
the  thickness  is  about  10  per  cent,  in  the  green  to  ultra-violet  regions 


TRANSMISSION  OF  RADIANT  ENERGY 


inclusive.  Hence,  the  most  marked  effect  of  change  of  thickness  in 
Crookes  A  lies  in  the  increased  absorption  in  the  yellow-green  region ; 
i.  e.,  the  region  of  maximum  visibility  of  the  human  eye.  Further- 
more, the  change  in  tint  due  to  varying  thicknesses  of  glass  in  the  fin- 
ished product  is  practically  negligible  with  Crookes  A  and  Noviol  0. 

No  viol  0  shows  considerable  increase  (about  15  per  cent.)  in  the 
absorption  of  the  short  wavelengths  as  the  thickness  is  changed  from 
1  mm.  to  3mms.  Thickness  of  glass  has  but  little  effect  upon  the 
transmission  of  the  yellowish-green,  yellow,  orange  and  red  radia- 
tions in  the  case  of  Noviol  0.  Hence  this  glass,  by  increased  thick- 
ness, cuts  down  the  percentage  transmission  of  the  green,  blue,  violet 


7 


\ 


»w  WO 


Fig.  53 — Effect  of  thickness  on  the  transmission  of  B  Smoke.     (From  the 
American  Optical  Company.) 

and  ultra-violet  without  appreciably  affecting  the  transmission  of 
energy  lying  in  the  region  of  maximum  visibility. 

The  data  on  Smoke  B  and  Fieuzal  A,  as  plotted  in  Figures  53  and 
54,  speak  for  themselves.  In  the  case  of  Smoke  B,  in  the  region  from 
6500  to  4500  Angstroms  approximately,  a  change  of  thickness  from 
1  mm.  to  3  mms.  changes  the  transmission  from  50-60  per  cent,  to 
20-25  per  cent.  The  effect  of  thickness  is  less  marked  in  the  ultra- 
violet regions.  It  follows,  therefore,  that  thickness  has  a  marked  ef- 
fect upon  the  transmission  throughout  the  whole  of  the  visual  spectral 
region. 

Fieuzal  A  shows  that  the  effects  of  thickness  are  pronounced,  espe- 
cially in  the  region  from  5500  to  4000  Angstroms.  Here  again  the 
percentage  transmission  is  cut  in  half  by  a  change  of  thickness  from 
1  mm.  to  3  mms.  of  glass. 

The  general  conclusion  which  may  be  drawn  from  these  curves  is: 
Appreciable  changes  in  thickness — as  judged  from  the  standpoint  of 
ophthalmic  lens  manufacture — may  occur  in  lenses  and  prisms  made 


TRANSMISSION  OF  RADIANT  ENERGY 


55 


from  Crookes  A  and  Noviol  O  without  any  marked  change  in  the  per- 
centages of  various  kinds  of  radiant  energy  transmitted  by  these 
glasses.  Hence,  they  are  nearly  as  effective  in  their  transmissive 
and  absorptive  powers  when  made  up  in  lenses  having  thicknesses 
up  to  1  mm.  as  when  these  thicknesses  are  of  the  order  of  3  mms.  As 
a  result,  we  are  led  to  conclude  that  concave  or  convex  lenses  of  high 
powers  will  have,  when  made  up  in  Crookes  A  and  Noviol  0,  prac- 
tically the  same  effect  upon  the  character  of  the  luminous  energy 
ultimately  reaching  the  retina.  Therefore,  thicknesses  commonly 
used  in  ophthalmic  lens  manufacture  will  not  cause  any  noticeable 


Fig.  54 — Effect  of  thickness  on  the  transmission  of  A  Fieuzal.     (Permission  of  the 
American  Optical  Company.) 

variation  in  the  tint  or  affect  the  transmission  in  appreciable  amounts 
if  either  Crookss  A  or  Noviol  0  is  used. 

The  Infra-Red. 

The  fact  that  glassblowers  are  subject  to  a  special  form  of  cataract 
has  raised  the  question  as  to  whether  or  not  this  action  is  due  to 
radiant  energy  and  if  so,  whether  this  action  is  of  an  abiotic  or  thermic 
nature  or  whether  it  is  caused  by  ultra-violet  or  infra-red  radiations. 
Meyhofer  in  1886  examined  over  five  hundred  glassmakers  and  found 
about  12  per  cent,  affected  with  cataract.  The  cataract  almost  always 
appears  first  in  the  left  eye  which  is  the  more  exposed  to  the  energy 
from  the  molten  glass.  When  appearing  in  the  right  eye  first  Stein 
showed  that  the  glassblower  had  the  habit  of  turning  that  side  of  the 
face  toward  the  oven.  The  length  of  time  necessary  for  develop- 
ment of  the  cataract  is  not  exactly  known  but  evidently  comprises  sev- 
eral years.  The  workmen  also  develop  a  peculiar  rusty-brown  spot 
on  each  cheek,  generally  more  noticeable  on  the  left.  Hirschberg  states 
that  for  over  one  hundred  years  it  has  been  known  that  individuals 
exposed  ^o  intense  heat  and  light  are  especially  liable  to  cataract.  In 


56  TRANSMISSION  OF  RADIANT  ENERGY 

i 

the  case  of  the  glassblowers,  the  cataract  begins  as  a  rosette-like  or 
diffuse  opacity  in  the  cortex  at  the  posterior  pole  of  the  lens.  Later, 
striae  similar  to  those  of  senile  cataract  may  appear.  The  great  fre- 
quency, therefore,  with  which  glassblower 's  cataract  occurs,  its  rela- 
tively uniform  character,  and  the  fact  that  it  occurs  first  in  the  more 
exposed  eye  argues  for  the  statement  that  the  cataract  is  due  to  radiant 
energy  on  the  eye  itself.  The  further  questions  as  to  whether  the 
cataract  is  due  to  the  direct  action  of  the  light  upon  the  lens  or  upon 
the  eye  as  a  whole,  and  whether  it  is  due  to  abiotic  or  thermic  action 
are  not  so  easily  answered.  Cramer,  Stein  and  others  believe  that 
cataract  is  due  to  the  chemical  action  of  the  ultra-violet ;  Vogt  re- 
gards the  infra-red  as  chiefly  responsible. 

The  character  of  the  radiation  from  molten  glass  is  well  known. 
It  is  that  of  an  incandescent  body  of  about  1200°  to  1400°  C.  Crookes 
(Trans.  Royal  Soc.  Lon.,  1914)  says:  "As  far  as  one  can  judge  the 
temperature  at  the  melting  end  is  about  1500°  C.  and  at  the  work- 
ing end  decidedly  less — say  1200°  C."  It  is  certain  that  the  spectrum 
of  a  non-gaseous  body  at  this  temperature  does  not  include  any  of 
the  so-called  abiotic  radiation  since  the  extreme  limit  of  the  spectrum 
of  molten  glasses  as  found  by  investigators  is  3200  t.  m.,  and  estimates 
range  from  that  up  to  3350  t.  m.  Crookes  (I.  c.)  made  six  exposures, 
as  reported  in  his  paper,  with  different  times  of  exposure  and  found 
that  the  spectrum  extended  to  4520  t.  m.  after  twenty  minutes'  ex- 
posure and  from  that  time  of  exposure  on,  the  limit  of  spectrum  in 
the  ultra-violet  increased  to  3345  t.  m.  after  one  hundred  and  eighty 
minutes'  exposure.  Certain  it  is  that  an  exposure  of  three  hours  does 
not  permit  of  the  presence  of  much  ultra-violet  radiation.  Also,  with- 
out doubt,  abiotic  action  cannot  be  traced  beyond  about  3100  t.  m. 
Furthermore,  the  radiation  of  a  body  at  such  temperatures  is  rela- 
tively weak  all  through  the  ultra-violet  (vide  Figure  4).  The  maxi- 
mum, according  to  Planck's  and  "Wein's  laws,  for  a  body  at  1300°  C. 
lies  far  in  the  infra-red,  while  the  energy  in  the  whole  visible  and 
ultra-violet  part  of  the  spectrum  is  less  than  one  per  cent,  of  the 
total.  Hence,  to  ascribe  injurious  effects  to  the  visible  or  ultra-violet 
radiations  without  the  elimination  of  the  99  per  cent,  of  infra-red 
radiation  would  be,  on  its  face  at  least,  to  lose  all  sense  of  the  pos- 
sible correlation  of  cause  and  effect. 

These  possible  thermal  effects  on  the  eyes  and  this  abundance  of 
infra-red  radiation  are  of  significance  in  those  who  engage  in  such 
vocations  as  glassblowing  and  industries  in  which  welding  under 
powerful  arcs  is  common.  We  are  not  desirous  at  this  point  of  enter- 
ing into  an  account  of  the  various  arguments  and  experiments  for 


TRANSMISSION  OF  RADIANT  ENERGY  57 

and  against  the  view  that  the  infra-red  radiations  produce  deleterious 
actions  upon  the  eye:  we  shall  simply  give  the  results  of  various  in- 
vestigators as  to  the  absorption  by  various  glasses  in  the  infra-red. 

Sir  William  Crookes  appears  to  have  been  the  first  to  systematically 
engage  in  the  development  of  glasses  highly  absorptive  in  the  infra- 
red. His  experimentation  was  carried  on  in  connection  with  the 
Glass  "Workers  Cataract  Committee  of  the  Eoyal  Society,  and  con- 
sisted in  the  finding  of  the  effects  upon  the  ultra-violet,  the  visible 
and  the  infra-red  of  the  addition  of  small  quantities  of  metals  such 
as  cerium,  chromium,  cobalt,  copper,  iron,  lead,  manganese,  uranium, 
neodymium,  and  so  forth,  to  the  raw  soda  flux.  He  developed  a  glass 
No.  246,  consisting  of  90  per  cent,  raw  soda  flux,  10  per  cent,  ferrous 
oxalate  (a  small  quantity  of  red  tartar  and  powdered  wood  charcoal 
was  added  to  prevent  oxidation)  of  a  sage-green  in  color  which  cut 
off  ultra-violet  down  to  3800  t.  m.,  gave  a  heat  absorption  of  98  per 
cent,  and  transmitted  27.6  per  cent,  of  the  incident  light.  Another 
glass,  No.  217,  prepared  from  fused  soda  flux  96.8  per  cent.,  ferroso- 
ferric  oxide  2.85  per  cent,  and  carbon  0.35  per  cent.,  was  found  to 
cut  off  the  ultra-violet  below  3550  t.  m.,  to  cut  off  96  per  cent,  of  the 
heat  radiation  and  to  transmit  40  per  cent,  of  the  light. 

In  1917  W.  W.  Coblentz  and  W.  B.  Emerson  of  the  Bureau  of 
Standards  issued  a  paper  on  Glasses  for  Protecting  the  Eyes  from 
Injurious  Radiations  (Technology  Papers,  Bureau  of  Standards,  No. 
93).  This  paper  deals  largely  with  the  protective  properties  of  glasses 
which  shield  the  eye  from  infra-red  rays.  In  order  to  discuss  their 
results  we  shall  follow  their  sub-divisions  of  subject  matter  accord- 
ing to  the  color  of  the  glasses.  The  figures  and  diagrams  accompany- 
ing this  discussion  are  from  the  paper  of  Cobl-entz  and  Emerson. 

Curve  A  (Figure  55)  shows  the  transmission  of  energy  by  the 
human  eye.  From  this  transmission  curve  it  will  be  noticed  that 
radiations  of  wavelength  greater  than  1.4  /*  cannot  reach  the  retina. 
In  fact,  because  of  the  presence  of  water,  which  is  very  opaque  to 
infra-red  rays,  but  little  radiation  of  wavelengths  greater  than  1.5  /* 
passes  through  the  cornea.  The  cornea  is  about  0.6  mm.  thickness. 
From  this  it  will  be  noted  that  about  97  per  cent,  of  the  energy 
radiated  from  a  furnace  at  1000°  to  1200°  C.  (vide  Curve  B,  Fig- 
ure 57)  is  absorbed  in  the  outer  portion  of  the  eye. 

Y ' ellmv-colored  glasses.  Curve  B,  Figure  55,  gives  the  transmis- 
sion of  a  Noviol  glass,  curve  C  that  of  an  orange  and  curve  D  that 
of  a  canary  glass  all  of  2  mm.  thickness.  Curve  E  is  that  of  a  color- 
less (or  white)  glass.  The  obstruction  of  these  yellow  glasses  is  but 
little  greater  than  that  caused  by  an  equal  thickness  of  colorless  glass. 


58 


TRANSMISSION  OF  RADIANT  ENERGY 


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Fig.  55 — A,  human  eye,  Corning  noviol  glasses;  B,  yellow  (thickness,  tz=2.05 
mm)  ;  C,  orange  (t=2.03  mm) ;  D,  canary  (t=1.85  mm) ;  E,  Corning  G  124  JA, 
blue-green  (t=1.5  mm).  (After  Coblentz  and  Emerson.  Permission  of  Bureau  of 
Standards.) 


Fig.  56— Crookes's  glasses;  A,  light  (t=1.96  mm);  B,  dark  (t=2.00  mm);  C, 
ferrous  No.  30,  sage-green  (t— 1.98  mm).  D,  Schott's  black  glass  (tm3.6  mm). 
E,  -white  crown  glass  (t=2.18  mm).  F,  blue-green  glass  (A.  O.  C.  Lab.  No.  59; 
t=1.93  mm).  (A  and'B  are  Crookes's  neutral-tint  glass.)  (After  Coblentz  and 
Emerson.  Permission  of  Bureau  of  Standards.) 


TRANSMISSION  OF  RADIANT  ENERGY 


59 


The  amount  of  infra-red  transmitted  by  such  a  glass  as  Noviol  is 
about  55  per  cent,  of  the  total  radiation  from  a  furnace  heated  to 
1000°  to  1100°  C. 

Crookes's  glasses.     Figure  56  gives  the  transmission  curves  for  a 
family  of  Crookes  glasses.     The  lighter  or  neutral  shades  absorb  but 


Fig.  57 — A,  C,  gold  glass;  B,  emission  of  black  body  (1050°  C) ;  D,  electric 
smoke  (red)  [ordinates=iemission  scale]  (t=2.52  mm).  (Permission  of  Bureau 
of  Standards.) 


O.O  1.0  2.O  3.O  V.O  5.O/1 

Fig.  58 — Corning  "New  Noviweld"  glasses:  Top  curve=shade  30  per  cent; 
second  curve  from  top=shade  3;  next  lower  curve=shade  4%;  bottom,  eurvez= 
shades  6  and  7.  Thickness  of  glasses,  2.2  mm.  (Permission  of  Bureau  of  Stand- 
ards.) 

little  more  than  white  crown  glass.  Curve  C  gives  the  transmission 
of  Crookes  sage-green  glass  (marked  Ferrous  No.  30).  The  trans- 
mission in  the  green  is  about  45  per  cent.,  while  in  the  infra-red  the 
maximum  transmission  is  about  11  per  cent,  and  this  for  only  a 
narrow  spectral  region.  Curve  F  is  from  a  blue-green  glass  (marked 


60  TRANSMISSION  OF  RADIANT  ENERGY 

Lab.  No.  59  from  the  American  Optical  Co.)  which  transmits  about 
43  per  cent,  in  the  visible.  In  the  infra-red  it  is  more  opaque  than 
the  sage-green  just  described. 

Pfund  gold-plated  glass.  Metals  are  extremely  opaque  to  infra- 
red radiations.  In  the  visible  spectrum  gold  has  a  region  of  low 
reflectivity  and  great  transparency  in  the  region  of  0.5  p  (green). 
This  property  naturally  suggests  itself  as  an  excellent  method  of 
eliminating  all  the  infra-red  by  covering  white  spectacle  glass  with  a 
thin  layer  of  gold.  "The  high  reflecting  power  (metallic  reflection 
of  60  to  80  per  cent,  as  compared  with  the  vitreous  reflection  of  about 
4  per  cent,  from  glass)  makes  it  desirable  to  mount  these  gold-plated 
glasses  in  a  hood  (goggles)  which  prevents  reflection  of  light  from 
the  rear  surface  of  the  film  into  the  eye.  Curves  A  and  C  of  Figure 
57  show  that  the  gold-plated  glass  is  an  extremely  effective  means 
of  shielding  the  eye  from  the  infra-red  rays.  At  1.5  /*  the  trans- 
mission is  only  about  2  per  cent.,  while  beyond  2  /*  the  transmission 
is  less  than  1  per  cent.  This  Pfund  glass  obstructs  99  per  cent,  of 
the  infra-red  rays  emitted  by  a  furnace  heated  to  1050°  C.  The 
Pfund  gold-plated  glass,  made  by  the  American  Optical  Co.,  is  put 
out  as  a  gold  film  deposited  upon  Crookes  A." 

Blue-green  glasses.  Curve  E  of  Figure  55  shows  the  transmission 
of  a  bluish-green  glass  (Corning  G  124  JA)  which  has  fifty  per  cent, 
transmission  in  the  green  and  a  very  low  transmission  in  the  infra- 
red, This  sample  transmits  only  6  per  cent,  of  the  infra-red  radia- 
tion from  a  furnace  at  1050°  C. 

Greenish-brown  glasses.  These  glasses  protect  from  the  ultra-violet 
and  to  some  extent  from  the  infra-red  rays.  The  maximum  trans- 
mission in  the  visible  is  about  27  per  cent.  The  coloring  matter  is 
effective  in  its  absorption  at  1  ^  but  beyond  3  fi  the  transmission  is 
about  as  high  as  in  uncolored  glass. 

Black  glasses.  Curve  D  in  Figure  56  gives  the  transmission  of  a 
sample  of  Schott's  black  glass:  the  transmission  in  the  visible  spec- 
trum is  quite  uniform  and  amounts  to  about  0.5  per  cent.  The  sample 
used  in  Figure  56  transmitted  little  beyond  3  p.  although  a  lighter 
colored  shade  was  transparent  to  5  p..  This  sample  transmits  about  18 
per  cent,  of  the  infra-red  radiation  emitted  by  a  black  body  heated 
at  1050°  C. 

Noviweld  glasses.  As  illustrated  in  Figure  58,  the  infra-red  trans- 
mission of  modern  noviweld  glasses  is  practically  suppressed.  The 
darkest  shades  transmit  only  about  1  per  cent,  of  the  infra-red  radia- 
tion emitted  from  a  furnace  heated  to  about  1000°  C.  The  trans- 


TRANSMISSION  OF  RADIANT  ENERGY 


01 


mission  in  a  rather  selective  region  with  a  maximum  at  about  0.5  /u 
(yellowish-green  region)  is  rather  marked. 

The  transmission  curves  in  the  visible  and  infra-red  regions  for 
the  French  Fieuzal  and  the  German  Hallauer  glasses  are  shown  in 
Figure  59. 

It  will  be  noted  that  glasses  which  absorb  highly  in  the  infra-red 
have  either  a  low  transmission  throughout  the  visible  spectrum  or  have 
the  transmission  band  shifted  into  the  green  or  blue. 


Fig.  59— A,  Lab.  No.  61,  A.  O.  C.  (1=2.09  mm)  ;  B,  Fieuzal  glass,  shade  B 
(t— 2.04  mm);  C,  Hallauer  glass  (t=1.41  mm).  (Permission  Bureau  of  Stand- 
ards.) 

Coblentz  and  Emerson  say,  by  way  of  conclusion,  that  "For  shield- 
ing the  eye  from  infra-red  rays  deep-black,  yellowish-green,  sage-green, 
gold-plated  and  bluish-green  glasses  are  the  most  serviceable.  For 
working  near  furnaces  of  molten  iron  or  glass,  if  considerable  light 
is  needed  a  light  bluish-green  or  sage-green  glass  is  efficient  in  abstract- 
ing the  infra-red  rays.  For  working  molten  quartz,  operating  oxy- 
acetylene  or  electric  welding  apparatus,  search-lights,  or  other  intense 
sources  of  light,  it  is  important  to  wear  the  darkest  glasses  one  can 
use,  whether  black,  green  (including  gold-plated  glasses)  or  yellowish- 
green,  in  order  to  obstruct  not  only  the  infra-red  but  also  the  visible 
arid  the  ultra-violet  rays." 

Figure  60  gives  a  good  comparative  set  of  curves  for  the  trans- 
mission of  the  eye  media,  yellow  glass,  sage-green,  neutral  tint,  gold- 


TRANSMISSION  OF  RADIANT  ENERGY 

plate,  greenish-brown,  black  and  blue-green  glasses  and  the  emission 
curves  of  a  black  body  at  1050°  C. 

A  detailed  examination  of  the  infra-red  transmission  of  a  consider- 
able number  of  glasses  on  the  market  and  used  for  spectacle  lenses 
was  made  in  1917  by  A.  "W.  Smith  and  C.  Sheard.  The  results  of 
their  investigations  are  published  in  the  Journal  of  the  Optical 
Society  of  America.  (Vol.  II-III,  Jan.  1919).  The  Hilger  infra-red 


100 


Fig.  60 — A,  eye  media  (Fig.  1,  A)  ;  B,  yellow  glass  (Fig.  1,  B)  ;  C,  sage  green 
(Fig.  2,  C)  ;  D,  neutral  tint  (Fig.  2,  B)  ;  E,  gold  plate  (Fig.  3,  A)  ;  F,  greenish- 
brown  (Fig.  4,  A)  ;  G,  black  glass  (Fig.  2,  D)  ;  H,  blue-green  (Fig.  1,  E)  ;  I, 
black  body  (1050°  C).  (After  Coblentz  and  Emerson.  Permission  of  Bureau  of 
Standards.)  • 

spectrometer  was  used  for  these  investigations.  The  width  of  slits 
used  in  these  experiments  was  such  as  to  give  a  range  of  spectrum 
at  the  thermopile  of  between  0.1  and  0.26  p..  A  Nernst  glower  served 
as  a  radiation  source.  Two  shutters  were  mounted  in  front  of  the 
spectrometer  slit;  one  of  these  carried  the  specimen  of  glass  to  be 
studied,  the  other  entirely  screened  the  slit  from  the  radiation  of 
the  glower.  To  get  a  measure  of  the  energy  transmitted  by  a  piece 
of  glass  for  a  particular  wavelength,  the  deflection  of  the  galvanometer 
when  no  absorbing  medium  was  interposed  between  the  Nernst  glower 
and  the  spectrometer  slit  was  divided  into  the  corresponding  deflection 
of  the  galvanometer  when  the  radiation  passed  through  the  glass  plate 


TRANSMISSION  OF  RADIANT  ENERGY 


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Fig.  64 — Tnuisinission  of  various  ophthalmic  glasrcs  in  the  infra-red.    (After  Smil 
and  Sheard:  Courtesy  of  the  Journal  of  the  Optical  Society  of  America.) 


TRANSMISSION  OF  RADIANT  ENERGY  G7 

before  it  reached  the  spectrometer  slit.  Figure  61  gives  a  sample 
set  of  data  obtained;  the  upper  curve  represents  the  energy  distribu- 
tion from  the  Nernst  glower  as  measured  by  the  galvanometer  de- 
flections and,  the  lower  curve,  in  a  similar  manner,  measures  the 
energy  transmitted  by  the  sample  of  glass.  Figures  62,  63  and  64 
contain  a  whole  series  of  curves  so  related  in  general  that  various 
shades  of  the  same  colored  glass  are  in  sequence.  The  so  called  nactics 
— a  trade  name — and  ambers  occupy  the  whole  of  Figure  63.  The 
rather  noticeable  selective  absorption  in  the  region  of  0.75  to  1.75  p. 
(broadly)  exhibited  by  the  yellow  glasses  (noviol,  nactic  and  ambers) 
is  evidently  characteristic  of  such  glasses.  The  heat  transmission  of 
the  German  Euphos  glass  is  greater  than  for  any  of  the  common 
ambers  as  tested  except  possibly  that  marked  "First  Amber"  which 
it  closely  resembles. 

In  Figures  62-64  the  wavelengths  are  plotted  on  the  horizontal  axis ; 
the  percentages  of  transmission  on  the  vertical  axis.  In  order  to 
economize  space  the  vertical  axis  has  been  so  lettered  that  the  80 
per  cent,  of  one  curve  coincides  with  the  zero  point  of  the  curve  lying 
immediately  above  it. 

It  seems  useless  to  attempt  to  present  the  transmission  properties 
of  all  the  numerous  glasses  which  are  obtainable  under  different  trade 
names  but  which  have  a  characteristic  color.  The  color  of  the  same 
kind  of  glass  may  differ  somewhat  for  different  melts  and  for  different 
parts  of  the  same  melt.  This  may  have  a  marked  effect  upon  the 
visible  spectrum  but  does  not  in  general  affect  the  coloring  matter. 
Certain  colors  of  glasses  are  difficult  to  match.  There  is,  in  our 
opinion,  too  great  a  variety  of  colored  lenses  and  a  strict  standardiza- 
tion of  these  is  to  be  hoped  for.  At  any  rate,  as  Verhoeff  and  Bell 
write,  "Perhaps  the  chief  benefit  of  the  agitation  that  has  taken  place 
within  the  last  decade  on  the  possible  *  *  *  dangers  of  the 
ultra-violet  has  been  the  bringing  into  prominence  of  the  new  types 
of  protective  glasses.  These,  intended  primarily  for  the  elimination 
of  the  ultra-violet  rays,  have  tended  to  types  of  selective  absorption 
which  give  advantageous  results  in  modifying  the  visible  light,  which 
is  really  the  chief  object  of  concern  of  the  ophthalmologist." 

CHAPTER   IV.       TRANSMISSION    OF    THE    OCULAR   MEDIA. 

In  1908  Parsons  of  London,  England,  in  conjunction  with  E.  C- 
Baly,  F.  R.  S.,  an  authority  on  spectroscopy,  and  E.  F.  Hendersoa 
carried  out  some  investigations  on  the  absorption  spectra  of  the 
cornea,  lens  and  vitreous  of  the  rabbit's  eye.  Later  Parsons  andi 
Martin  made  a  more  extensive  study,  the  results  of  which  appeared  ic 


08  TRANSMISSION  OF  RADIANT  ENERGY 

the  Journal  of  the  American  Medical  Association  (Vol.  LX,  page 
2027,  1910).  These  researches  were  antedated  in  some  particulars  by 
those  of  Birch-Hirschfeld,  Halla.uer  and  Sehanz  and  Stockhausen. 
The  accompanying  diagrams  (Figures  65-69)  are  from  originals  taken 
at  the  Imperial  College  of  Science  and  Technology  in  the  laboratories 
of  Sir  William  Abney  and  Professor  Fowler,  F.  R.  S. 

The  Ultra-violet  and.  the  Visible. 

In  the  research  under  discussion    ;xperiments  were  made  to  de- 
termine the  precise  limits  within  whic?i  the  short  wavelengths  of  light 


Fig.  65 — The  transmission  of  tht  cornea.     (After  Parsons.) 


Fig.  66 — Spectrograms  showing  the  transmission  of  the  crystalline  lens. 

(After  Parsons.) 

were  absorbed  by  the  refractive  media  of  the  eye  and  the  effects  on 
these  limits  of  keeping  the  media  some  hours  after  the  death  of  the 
animal.  The  media  were  mounted  in  cells  with  parallel  sides  In 
the  case  of  the  cornea  and  vitreous  the  cell  was  placed  close  t<>  the 
slit  of  the  spectroscope.  The  lens  was  dealt  with  in  two  ways: — 
{!)  Suspended  in  normal  saline  and  placed  at  a  distance  from  the 
slit  greater  than  its  focal  length,  so  that  a  blurred  image  of  the  source 
of  light  was  thrown  on  the  slit.  In  this  way  horizontal  lines  were 
avoided  on  the  resulting  photographs  and  the  possibility  of  stray 
light  entering  between  the  cell  and  the  slit  was  prevented.  (2)  A  thin 
layer  of  lens  substance  was  squeezed  out  flat  between  the  parallel  sides 


TRANSMISSION  OF  RADIANT  ENERGY  69 

of  the  cell ;  this  was  done  to  eliminate  any  possible  apparent  absorp- 
tion due  to  the  shape  of  the  lens. 

All  of  the  media  were  found  to  be  uniformly  permeable  to  rays 
between  the  wavelengths  6600-3900  t.  m.  For  the  ultra-violet  rays  the 
iron  arc  was  the  source  and  quartz  was  used  throughout.  Plates  con- 
taining no  dyes  and  giving  no  absorption  bands  were  used.  The 
results  obtained  by  Parsons  agree  closely  with  those  obtained  by 
Schanz  and  Stockhausen  and  Birch-Hirschfeld.  The  shortest  interval 
between  the  death  of  the  animal  and  the  taking  of  observations  was 
three  minutes.  Observations  were  also  made  on  the  vitreous  one  hour, 
lens  five  hours  and  cornea  several  hours  after  the  death  of  the  animal. 


Fig.  67 — Photograph  showing  the  spectra  of  the  crystalline  leng  in  normal  saline 
and  the  cortex.     (After  Parsons.) 

The  results  obtained  were  identical  with  those  from  fresh  specimens. 
The  conclusions  to  be  drawn  from  Figures  65-69  are : — 

"Cornea.  The  cornea  was  found  to  offer  no  resistance  to  rays 
of  wavelength  longer  than  2950  t.  m.,  but  all  those  beyond  this  limit 
were  completely  cut  off. 

Lens,  (a)  Suspended  in  normal  saline.  Rays  of  wavelengths  less 
than  3500  t.  m.  are  absorbed  completely.  The  line  is  not  a  sharp  one, 
the  absorption  commencing  at  about  4000  t.  m.  (b)  Squeezed  to 
different  thicknesses.  The  absorption  varies  pari  passu  with  the  thick- 
ness of  the  layer  of  lens  substance. 

Vitreous.  The  vitreous  in  a  layer  T3^  inch  thick  shows  a  broad 
absorption  band  extending  from  2800  to  2500  t.  m.,  with  a  maximum 
at  2700  t.  m.  The  margins  of  the  band  are  ill-defined." 

It  would  be  unsafe,  however,  to  apply  those  results  obtained  with 
rabbits'  eyes  directly  to  the  human  eye  without  further  investigations. 
The  results  of  Schanz  and  Stockhausen  (various  papers  in  Klin. 


70  TRANSMISSION  OF  RADIANT  ENERGY 

Monatsbl.  f.  Augenh.)  on  the  transmission  of  the  cornea  and  the 
vitreous  of  a  calf's  eye  are  confirmatory.  Birch-Hirschfeld  also 
investigated  the  transmission  properties  of  the  media  of  the  eyes  of 
calves,  pigs  and  oxen.  He  discovered  that  there  was  little  difference 
in  the  absorption  of  the  ultra-violet  by  these  cornese,  giving  the  limit 
as  3060  t.  m.,  somewhat  higher  than  for  the  rabbit  and  considerably 
more  than  that  of  ordinary  glass.  Birch-Hirschfeld  found  the  limit 
of  absorption  to  be  3000  t.  m.  for  a  layer  of  vitreous  1  cm.  thick. 
Greater  differences  which  cannot  be  overlooked  were  found  with  vari- 
ous lenses.  The  limits  of  transmission  of  the  rabbits'  lenses  varied 
between  3300  t.  m.  and  3900  t.  m.  For  the  pig's  lens  the  average 
limit  was  3300  t.  m.  with  variations  of  about  150  t.  m. ;  for  the  calf's 


Fig.  68 — Spectrograms  showing  the  transmission  of  the  ultraviolet  by  the  vitreous. 

(After  Parsons.) 

lens  at  3280  t.  m.  with  variations  of  120  t.  m.  and  for  the  ox  lens  from 
2700  to  4000  t.  m.  Schanz  and  Stockhausen  examined  the  cornea  and 
lens  of  a  child  who  had  glioma.  The  cornea  absorbed  up  to  about 
3000  t.  m.  and  the  lens  about  the  same.  In  a  certain  case  of  injury 
the  corneal  absorption  was  about  the  same  but  that  of  the  lens  was 
much  greater,  i.  e.,  up  to  about  3500  t.  m.  Hallauer  (Klin.  Monatsbl. 
f.  Augenh.  1909)  found  that  the  corneal  and  vitreous  absorptions  in 
the  eye  of  a  man  extended  to  2950  t.  m.  He  examined  the  lenses  from 
a  considerable  number  of  individuals  of  different  ages  and  reached 
some  valuable  conclusions.  He  found  the  limits  of  absorption,  on  the 
whole,  dependent  upon  age  with  some  individual  variations  due  to 
thickness,  color  and  consistency.  General  conditions  of  disease  also 
introduce  a  disturbing  factor  which  must  be  taken  into  account  in 
Schanz  and  Stockhausen 's  case  of  glioma.  In  babies  and  young 
children  the  absorption  extends  to  about  4000  t.  m.  With  this  absorp- 
tion, however,  is  combined  an  inability  to  absorb  .rays  from  3000  to 
3100  t.  m.  This  transparent  band  is  said  to  persist  up  to  about  the 
twentieth  year  and  may  be  more  extensive  in  certain  debilitated  con- 


TRANSMISSION  OF  RADIANT  ENERGY  71 

ditions.  Rather  peculiarly,  with  the  loss  of  this  band  after  twenty, 
it  is  claimed  that  the  limit  of  absorption  drops  from  4000  t.  m.  to 
about  3770  t.  m.  With  advancing  age,  however,  the  crystalline  lens 
becomes  more  and  more  'yellow'  as  a  general  rule  and  therefore  its 
absorptive  powers  reach  down  into  the  violet,  extending  even  up  to 
4200  t.  m.  Extreme  debility  from  disease  diminishes  absorption  to 
a  minimum  of  3750  t.  m. 

All  of  these  investigations  show  that  the  lens  has  a  powerful 
capacity  for  absorbing  ultra-violet  light  in  the  region  roughly  com- 
prised between  3000  to  3800  t.  m.  The  fact  is  very  easily  and  strik- 
ingly demonstrated  by  the  strong  fluorescence  which  occurs  when  these 
rays  strike  it.  Schanz  and  Stockhausen  attribute  this  fluorescence  to 


Fig.  69 — Transmission  of  the  ultraviolet  by  the  vitreous.     (After  Parsons.) 

the  rays  between  4000  t.  m.  and  3500  t.  m.  In  the  paper  (Illuminating 
Engineer,  1910)  on  Glare — Its  Causes  and  Effects,  by  Stockhausen, 
we  find  this  statement: — "Now  the  ultra-violet  rays  between  3750 
t.  m.  and  3200  t.  m.  are  strongly  absorbed  by  the  eye-lens  and  those 
between  4000  t.  m.  and  3750  t.  m.  are  for  the  most  part  altered  into 
fluorescence  light  in  the  lens.  Violet  rays  also,  as  Schanz  and  Stock- 
hausen have  shown,  generally  contribute  to  some  extent  to  this  change. 
Now,  in  general,  it  is  only  those  rays  which  are  absorbed  by  any  sub- 
stance which  exert  a  chemical  action  upon  it  and  we  are,  therefore, 
justified  in  supposing  that  it  is  the  ultra-violet  rays  which  are  ab- 
sorbed by  the  lens  that  produce  the  effect  referred  to  above.  In  addi- 
tion, the  conversion  of  all  ultra-violet  rays  and  a  portion  of  violet 
light  into  visible  light  by  fluorescence  indicates  a  transformation  of 
energy  and  in  the  course  of  years  may  produce  the  injury  to  the  eye 
known  as  cataract."  But,  as  pointed  out  by.Helmholtz,  a  fluorescent 
body  always  strongly  absorbs  those  rays  which  induce  the  fluores- 
cence. Hence  the  chief  role  must  be  allotted  to  rays  between  3500  t.  m. 
and  3000  t.  m.,  for  those  from  4000  t.  m.  to  3500  t.  m.  are  absorbed 


72  TRANSMISSION  OF  RADIANT  ENERGY 

to  some  extent  by  the  lens.  Also,  as  pointed  out  by  Helmholtz  and 
Stokes,  the  light  causing  a  fluorescence  is  of  a  'shorter  wavelength 
than  that  of  the  emitted  fluorescence.  The  investigation  of  this 
fluorescence  of  lenses  is  not  unattended  with  complicating  features,  es- 
pecially those  due  to  fluorescence  of  the  observer's  own  lenses. 

The  Infra-red. 

The  general  absorption  of  the  eye  media  has  been  studied  by  Asch- 
kinass  (Arm.  der  Phys.  und  Chem.,  Vol.  55,  1895)  in  connection  with 
his  determination  of  the  absorption  spectrum  of  fluid  water.  He  found 
that  the  transmission  of  the  media  of  the  eye  for  radiant  energy  in 
general  was  closely  similar  to  that  of  water  in  a  layer  of  equal  thick- 
ness. The  large  proportion  of  water  in  these  media  would,  of  course, 
suggest  a  similarity  and  Aschkinass  found  the  characteristic  absorp- 
tion bands  of  water  in  the  experiments  on  the  eyes  of  cattle  and  some 
control  experiments  on  the  human  eye.  The  only  notable  discrepancy 
was  in  finding  a  considerably  higher  absorption  in  the  cornea  than 
would  be  warranted  by  its  water  equivalent.  This  Aschkinass 
ascribes  chiefly  to  a  film  forming  very  rapidly  over  the  surface  of 
the  dead  cornea. 

Hartridge  and  Hill,  working  in  the  Physiological  Laboratory  of 
Cambridge,  England,  have  carried  out  some  important  investigations 
upon  the  transmission  of  infra-red  rays  by  the  media  of  the  eye.  This 
work  is  published  in  the  Proc.  of  the  Royal  Society  of  London,  Series 
B,  Vol.  89,  1917.  These  investigators  used  a  constant  deviation 
Hilger  spectrometer:  in  place  of  the  eye-piece  in  the  telescope  there 
was  inserted  an  adjustable  vertical  slit  behind  which  was  mounted 
a  delicate  thermopile  of  ten  bismuth-silver  elements.  This  thermopile 
was  connected  to  a  Paschen-or  Broca  galvanometer  and  the  energy 
falling  upon  the  thermopile  was  measured  by  its  deflection.  The 
whole  telescope  was  protected  from  radiant  and  convected  heat  by  a 
silvered  vacuum  flask,  the  mouth  of  which  was  plugged  with  cotton. 
The  light  source  was  a  single  vertical  Nernst  filament.  The  spectral 
examination  of  the  aqueous  and  vitreous  offered  no  great  difficulty 
mechanically,  since  they  could  be  held  in  parallel-faced  glass  or  quartz 
containers.  With  the  lens  and  cornea  this  is  not  the  case.  Two 
methods  are  available:  first,  to  dry  the  lenses  superficially  and  then 
to  squeeze  them  into  a.  small  trough.  This  method  is  not  highly  suc- 
cessful since  the  differences  in  refractive  indices  of  various  zones  of 
the  eye  lens  cause  a  series  of  confused  images  of  the  light  source.  A 


TRANSMISSION  OF  RADIANT  ENERGY  73 

second  and  better  method  is  the  immersion  of  the  uninjured  lens  in 
some  fluid  of  suitable  refractive  index  that  will  neutralize  the  con- 
vergence exerted  by  the  lens  on  a  parallel  beam  of  light  passing 
through  it.  Hartridge  and  Hill  found  that  carbon  tetrachloride  was 
most  suitable  for  this  purpose;  it  has  no  absorption  bands  over  the 
region  to  be  investigated,  it  does  not  precipitate  the  proteids  of  the 
lens  and  has  marked  antiseptic  properties.  An  extensive  series  of 
experiments  proved  that  lens  preparations  made  in  this  way  gave  the 
absorption  bands  corresponding  to  those  of  water.  The  absorption 
curve  of  water  in  comparison  with  the  lens  of  the  eye  is  shown  in 
Figure  70.  It  will  be  apparent  to  the  reader  that  a  superposition  of 
two  curves  showing  the  amounts  of  energy  of  different  wave-lengths 
transmitted  could  not  occur  unless  "equivalent"  thicknesses  of  water 
and  media  were  taken.  The  following  table  gives  such  data: 

TABLE   III. 

Water  Equivalent 

Structure                              Thickness.  Index,  (percent.),  thickness  of  water. 

Cornea    1.15  mm.  1.377  90 .  1.04  mm. 

Aqueous    2.5  1.355  99  2.38 

Lens  center 84  .... 

Lens  cortex   4.05  1.39  92  3.35 

Vitreous    15.00  1.340  96  14.4 

Such  information  is  of  great  value  since  it  permits  the  substitution 
of  the  equivalent  thickness  of  water  in  experimental  work,  thus  remov- 
ing the  tedium  and  uncertainty  in  results  due  to  a  time  factor  neces- 
sarily involved  in  dealing  with  anatomical  media. 

The  table  as  given  by  Luckiesh  (Electrical  World,  Oct.,  1913)  differs 
somewhat  from  the  figures  as  given  by  Hartridge  and  Hill.  Luckiesh 's 
data  are  as  follows: 

Media.  Equivalent  cms.  of  water. 

Cornea    0.06 

Aqueous  humor 0.34 

Crystalline  lens  0.42 

Vitreous  humor 1.46 

Total  eye  2.28 

The  very  important  question  arises :  In  what  amounts  do  the  infra- 
red radiations  of  different  wave-length  gain  access  to  the  deeper  struc- 
tures of  the  eye  ?  In  other  words,  What  is  the  energy  density  in  the 
eye  media?  The  answer  to  this  question  has  been  undertaken  by 
Luckiesh  (Elec.  World.  1913)  and  by  Hartridge  and  Hill  (Proc.  Roy. 


1  TRANSMISSION  OF  RADIANT  ENERGY 

Soc.  of  London,  1917).     The  intensity  of  radiation  after  traversing 
any  depth,  d,  can  be  computed  from  the  following  equation : 

P  =  le  - ad 
where  I  and  /'  are  the  original  and  final  intensities  respectively,  e  is 


/  0,000 

WAVE     LENGTH 


Fig.  70  —  A  comparison  of  the  absorption  curves  of  water  and  the  crystalline  lens 
and  the  aqueous  humour  in  the  infra-red  region.     (After  Hartridge  and  Hill.) 

the  base  of  the  Naperian  logarithms  and  a  is  the  extinction  coefficient. 
This  can  be  further  simplified  for  purposes  of  calculation  thus: 


where  T  is  the  transmission  coefficient.  If  I  be  taken  as  unity,  then 
the  value  of  /'  is  equal  to  that  of  the  transmission  coefficient. 
Aschkinass  gives  in  his  paper  a  table  of  extinction  coefficients  for 
pure  water  from  0.45  p.  to  8.49  /*.  Hence  it  is  possible  to  compute  the 


TRANSMISSION  OF  EADIANT  ENERGY 


75 


transmissions  of  the  various  eye  media  within  this  range.  Aschkinass 
did  this  for  the  whole  eye :  the  transmissions  of  various  layers  of  water 
corresponding  to  the  eye  media  according  to  Luckiesh  (Elect.  World, 
1913)  are  given  in  the  curves  of  Figure  71.  The  first  curve,  that  of 
the  equivalent  cornea,  indicates  the  percentage  of  heat  energy  trans- 
mitted by  the  cornea  of  that  incident  upon  the  cornea;  the  second 
curve  shows  the  percentage  of  heat  energy  reaching  the  anterior  sur- 


71 


l.g  1.1  1.6 

WdrCL CNOTH 


g.o          t.a 


Fig.  71 — Transmission  of  various  layers  of  water  corresponding  to  the  eye  media. 
(Courtesy  of  M.  Luckiesh.) 

face  of  the  lens  of  that  incident  on  the  cornea,  and  so  forth.  Table  IV 
gives  the  set  of  experimental  data  obtained  by  Hartridge  and  Hill 
(Proc.  Roy.  Soc.  1917)  : 


TABLE   IV. 


Wavelength. 

7000 

7500 

8000 

8500 

9000 

9500 

9750 
10000 
10500 
11000 
11500 
12000 
12500 
12750 
13000 


Equivalent 
cornea. 
97.5 
97.5 
97.5 
97.5 
97.2 
94.4 
93.6 
94.5 
96.6 
95.9 
89.4 
86.4 
87 
87.3 
85.4 


Equivalent 

Equivalent 

cornea  and 

cornea,  aqueous 

aqueous. 

and  lens. 

95 

95 

95 

94.6 

94.5 

93.6 

94.2 

93 

93.6 

91.9 

85.4 

76.2 

83.1 

72.5 

85.8 

'  77.2 

93 

89 

90 

85.1 

71.5 

53.2 

63.7 

42.2 

65.7 

44.9 

65.6 

44.8 

61 

37.7 

Equivalent 
eye. 
94.3 
91.3 
89.6 
89 
86.1 
48 
41.2 
50.3 
77.6 
67.7 
15.9 
7.9 
9.5 
10.6 
6.55 


76 


TRANSMISSION  OF  RADIANT  ENERGY 


Wavelength. 
13500 
14000 
14500 
15000 
15500 
16000 
16500 
17000 
17500 
18000 
18500 
19000 
20000 
21000 
22000 


Equivalent 

Equivalent 
cornea  and 

Equivalent 
cornea,  aqueous 

Equivalent 

cornea. 

aqueous. 

and  lens. 

eye. 

75 

36.4 

13.4 

0.24 

23.5 

0.7 

5.5 

.... 

12.9 

1.2 

28 

1.38 

48.2 

8.7 

00.73 

53.3 

12.2 

1.44 

51.4 

10 

0.95 

43.5 

5.6 

0.3 

20.3 

0.4 

4.9 

2.0 

4.4 

7.6 

5 

Soeo 


/o.ooo 

W/WE    LENGTH 


Fig.  72 — Curves  showing  the  percentages  of  infra-red  radiation  absorbed  by  the 
media  specified  of  the  amount  of  energy  incident  upon  the  medium  named. 
(After  Hartridge  and  Hill.) 

• 

These  tables  and  curves  show  that  there  is  practically  no  transmis- 
sion of  energy  of  wave-length  greater  than  23000  Angstroms.  Paschen 
(Wied.  Annalen,  Vol.  52,  1894)  showed  that  a  layer  of  water  0.03  mm. 
thick  transmitted  at  no  wave-length  more  than  twenty  per  cent,  of  the 
incident  energy;  a  layer  2  mm.  thick  would,  therefore,  be  totally 
opaque  for  wave-lengths  greater  than  23000  t.  m.  Furthermore,  an 
inspection  of  Figure  71  shows  that  heat  radiations  of  wave-lengths 
from  7000  to  9500  t.  m.  roughly  pass  into  the  eye  almost  unchecked 
and  that  a  great  deal  of  it  reaches  the  retina.  Figure  72  shows :  Curve 
I,  percentage  of  heat  energy  absorbed  by  the  cornea  of  that  incident 
upon  it;  Curve  II,  percentage  of  heat  energy  absorbed  by  the  iris  of 
that  incident  on  the  cornea  and  Curve  III  gives  the  percentage  of 
heat  energy  absorbed  by  the  lens  of  that  incident  on  the  cornea.  The 
curves  of  this  diagram  are  all  representative  of  absorption;  those  in 


TRANSMISSION  OF  RADIANT  ENERGY 


77 


Figure  71  give  traiismisswn.  It  will  be  noted  that  the  absorption  of 
the  iris  for  wave-lengths  ranging  from  5000  to  10000  t.  m.  approxi- 
mates 95  per  cent.  Hartridge  and  Hill  (I.e.}  say  that  the  iris  of  the 
ox  totally  obstructs  heat  radiation  of  every  wave-length  which  falls 
upon  it.  The  lens,  on  the  other  hand,  absorbs  of  the  radiation  which 
falls  upon  it  by  way  of  the  aperture  of  the  iris  only  about  twelve  per 
cent.  Roughly  stated,  it  can  be  said  that  four  times  the  amount  of 
energy  is  absorbed  per  unit  area  of  the  iris  as  is  absorbed  by  the  lens. 


100 


1.5 
WWCLENGTH 


£J 


3.0 


2.5 


Fig.   73 — Transmission  of  radiant  energy  from  a  1.25  watt-per-candle  tungsten 
lamp  through  various  layers  of  water.     (Courtesy  of  M.  Luekiesh.) 

Another  point  of  interest  is  to  apply  these  transmission  curves  to  the 
curves  representing  the  spectral  energy  distribution  of  black  bodies 
at  various  temperatures  and  also  to  those  of  various  illuminants. 
Figure  73  gives  the  transmission  of  radiant  energy  from  a  1.25  watt- 
per-candle-tungsten  lamp  through  various  layers  of  water.  (Luekiesh, 
Elect.  World,  1913.)  The  numbers  on  the  curves  represent  the  thick- 
ness of  water.  For  example,  the  percentage  of  total  energy  radiated 
from  the  carbon  lamp  and  which  is  transmitted  by  the  cornea  is  found 
by  obtaining  the  ratio  of  the  area  under  this  curve  (0.06  cm.)  to  the 
total  area  under  the  radiation  curve.  The  difference  between  this  and 
unity  gives  the  absorption  of  the  cornea.  These  percentages  are  found 
in  Table  V  and  plotted  in  Figures  74-76.  Figure  74  gives  the  per- 
centages of  total  black-body  energy  absorbed  by  the  various  eye  media. 
It  will  be  seen  that  for  the  cornea  these  percentages  rapidly  decrease 
with  increase  of  temperature  of  the  source,  but  much  less  rapidly  for 
the  aqueous,  while  the  percentages  of  absorbed  energy  are  at  a  maxi- 


78 


TRANSMISSION  OF  RADIANT  ENERGY 


mum  for  the  lens  and  vitreous  humor  at  about  3500°  K. 
energy  is  absorbed  in  the  outer  portion  of  the  eye. 


Most  of  the 


TABLE  v. 
Percentage  of  energy  absorption. 

Percentage  of  total  energy  absorbed  in 


Source 

Water  of  depth 
O!06cm.  0.04cm.  0.82cm.  2.28cm. 

Cornea 

Aqueous 
humor 

• 

9 
• 

^ 

Vitreous 
humor 

Black  body  2000°K. 
Black  body  2500  °K. 
Black  body  3000°K. 
Black  body  4000°K. 
Black  body  5000°K. 
4  w.p.c.  carbon  

68.8 
51.7 
38.5 
22.8 
13.0 
64.1 
50.4 

80.6 
63.6 
49.8 
31.7 
19.6 
77.3 
64.5 

83.8 
68.3 
55.7 
37.2 
23.4 
81.0 
70.5 

89.7 
76.7 
65.1 
45.9 
30.4 
87.9 
80.0 

68.8 
51.7 
38.5 
22.8 
13.0 
64.1 
50.4 

11.8 
11.6 
11.3 
8.9 
6.7 
13.2 
14.1 

3.2 
5.0 
5.9 
5.5 
3.8 
3.7 
6.0 

5.9 
8.4 
9.4 
8.7 
7.0 
6.9 
9.5 

1.25  w.p.c.  tungsten. 

Percentages  of  energy  absorbed  have  only  been  considered.  The 
data  can  be  reduced  to  that  of  finding  the  actual  watts  absorbed  per 
lumen.  In  Figure  76  are  plotted  the  values  of  watts  per  lumen  for 
the  black  bodies  at  various  temperatures.  Multiplying  these  values 


COR 

NC4 

60 
SO 
40 
30 

*° 
10 
0 

\ 

\ 

\ 

\ 

\ 

V 

\ 

k 

N 

\ 

\ 

\ 

4» 

tuna 

— 

K/r. 

lurtjfi 

-^  —  • 

—  . 

•  — 

Urn 

r 

000"        iOOO*           3000°            4000°          3000" 
4&3OLUTt  fCf^PCf(flTUK£  OF&iACK  BObf 

fig.  74 — Percentage  of  total  radiant  energy  absorbed  in  various  eye  media. 
(Courtesy  of  M.  Luckiesh.) 

by  the  corresponding  values  for  the  curves  of  Figure  74,  the  actual 
watts  absorbed  per  lumen  are  obtainable.  Figure  75  '  carries  these 
results.  Curve  a  represents  the  absorption  for  the  total  eye ;  curve  & 


TRANSMISSION  OF  RADIANT  ENERGY 


79 


that  of  the  cornea,  and  so  on.  These  curves  give  the  actual  power 
absorbed  in  the  eye  media  per  lumen  of  light  flux  in  the  entering  beam. 
All  of  the  data  show  that  the  outer  layer  of  the  cornea  absorbs  a  large 
portion  of  the  energy  which  is  not  active  in  producing  the  sensation  of 


3000'  *00<T  5000' 

TcrtrctjTMe  of  SLICK  Sour 


Fig.  7o — Watts  absorbed  in  the  eye  media  per  lumen  in  usual  percentage  of  light. 
(Courtesy  of  M.  Luekiesh.) 


\ 


X 


f.ZS  ri 
0.8*.  c 


0,40  c 
•-O.O6  c 


o.2^ 

o.eo 

k 

O.I  a  a 

a 

0.16  5 
0.14  ^ 
O./i  S 


4000'          SOOO* 


Fig.   76 — Percentage  of  total  radiated  energy  absorbed  in  the  various  layers  of 
water.     (Courtesy  of  M.  Luekiesh.) 

light.  Also,  as  is  to  be  expected,  the  absorbed  energy  per  lumen  of 
light  flux  incident  upon  the  retina  rapidly  decreases  with  an  increase 
of  temperature  of  the  source.  It  will  be  noted  that  about  thirty  times 
as  much  energy  is  absorbed  in  the  total  eye  per  lumen  of  tungsten 
light  as  per  lumen  of  light  from  a  black  body  at  5000°  C.  As  Luekiesh 


80  TRANSMISSION  OF  RADIANT  ENERGY 

says:  "This  same  ratio  would  hold  approximately  for  sunlight  if  it 
were  not  for  the  moisture  in  the  atmosphere  which  absorbs  much  of 
the  infra-red  rays  before  they  reach  the  eye.  This  is  perhaps  fortunate 
considering  the  enormously  greater  intensities  of  illumination  en- 
countered in  daylight."  For  instance,  according  to  F.  E.  Fowle 
(Astrophysical  Jour.  1913)  the  amount  of  percipitable  water  existing 
in  the  form  of  atmospheric  water  vapor  averages  about  0.7  cm. 

The  marked  difference  between  the  action  of  water  and  the  eye 
toward  the  infra-red  on  the  one  hand  and  the  ultra-violet  on  the  other 
hand  is  noteworthy.  The  eye  media  transmit  the  visible  and  infra-red 
rays  in  the  same  manner  as  water.  This  is  not  true  for  ultra-violet 
radiation.  "Water  is  transparent  to  short-wave  radiation  far  into  the 


Pig.  77 — Path  of  light  in  the  eye.     Small  object.      (Courtesy  of  M.  Luekiesh.) 

ultra-violet.    In  fact,  no  noticeable  absorption  has  been  found  for  any 
of  the  ultra-violet  radiation  from  the  mercury  arc  in  a  quartz  tube. 

The  question  of  energy  density  in  the  eye  media  using  sources  sub- 
tending large  and  small  solid  angles  has  been  discussed  in  a  paper  by 
Luekiesh  (Elect.  World,  Sept.  1915).  Figure  7,7  shows  the  path  of 
light  in  the  eye  when  a  small  object  is  looked  at,  while  Figure  78  gives 
the  path  of  light  in  the  eye  for  an  extended  object.  The  useful  beam 
of  radiation  included  within  a  solid  angle  of  120°  at  the  eye  is  shown 
by.  the  full  lines  in  Figure  78  when  the  eye  is  accommodated  for  rea- 
sonably near  vision.  If  the  object  that  is  being  viewed  be  illuminated 
with  the  same  density  of  radiation  of  the  same  spectral  character  as 
that  used  for  the  small  object  tests  at  distance,  it  is  obvious  that  the 
brightness  of  the  retinal  image  will  be  the  same  and  a  much  greater 
amount  of  energy  will  pass  through  the  pupillary  aperture.  The 
energy  density  would  thus  be  a  million  or  more  times  as  great  as  in  the 
case  of  the  more  extended  source.  This  is  shown  diagrammatically  in 
Figure  79  for  equal  energy  densities  at  the  retina — that  is,  for  equal 


TRANSMISSION  OF  RADIANT  ENERGY 


81 


brightnesses  of  the  retinal  images.    Curve  D  represents  the  condition 
for  the  extended  source  and  curve  E  for  the  small  source. 

In  this  paper  Luckiesh  says  by  way  of  summary :    ' '  It  is  shown  that 
when  viewing  luminous  objects  of  small  area   (subtending  a  small 


RCTIN* 


1 

*7 
I 

\X  I 

1  \ 

/I            1 

Fig.  78 — Path  of  light  in  the  eye.    Extended  object.     (Courtesy  of  M.  Luckiesh.) 

solid  angle)  there  is  no  serious  concentration  of  energy  in  the  eye 
media  until  the  retina  is  approached.  However,  when  viewing  ex- 
tended objects  (large  solid  angle)  there  is  a  relatively  much  greater 
energy  density  in  the  lens  and  anterior  parts  of  the  eye  than  in  the 


Suaeacc   OF  CORNEA,  en. 


Fig.  79 — Energy  density  in  the  useful  beams    of   light  from  sources  subtending 
large  (D)  and  small  (E)  solid  angles.     (Courtesy  of  M.  Luckiesh.) 


$2  TRANSMISSION  OF  RADIANT  ENERGY 

posterior  portions.  When  the  retinal  images  are  of  the  same  bright- 
ness, there  will  be  a  much  greater  energy  density  in  the  lens  when 
viewing  an  object  subtending  a  large  solid  angle  than  when  the  ob- 
ject subtends  a  small  "angle  if  the  spectral  character  of  the  illuminant 
and  the  intensity  of  the  illumination  are  the  same.  This  indicates 
that  large  sources  of  a  relatively  low  visual  brightness  might  be  effec- 
tive in  forming  cataract  or  causing  eye  fatigue  if  the  "absorption  of 
energy  theory"  is  correct.  In  fact,  if  the  deterioration  of  the  lens  is 
due  to  ultra-violet  rays,  the  latter  might  be  present  in  such  small 
amounts  as  to  appear  harmless,  but  when  it  is  recalled  that  the  energy 
density  in  the  lens  is  very  high  when  viewing  extended  objects,  such 
as  the  sky,  pavements,  large  surfaces  of  molten  glass,  metal,  etc.,  it 
appears  to  be  possible  that  the  ultra-violet  rays  might  be  present  in 
sufficient  amount  to  do  damage.  From  this  standpoint  sunlight,  owing 
to  the  greater  intensities  encountered,  appears  to  be  probably  as  effec- 
tive in  producing  cataract  and  eye  fatigue  as  ordinary  artificial  illumi- 
nants,  even  after  allowing  for  the  higher  luminous  efficiency  of  the 
former  and  the  absorption  of  energy  by  the  water  vapor  present  in  the 
atmosphere." 


INDEX 


Abney 6,  13,  67 

Absorption  of  Energy  Curves  of 

Eye 75-82 

American  Optical  Co.  Transmission 

Curves 41-42,  52-55 

Angstrom 2,3 

Aschkinass 14,  71,  74 

v.  Baeyer 6,  13 

Band  Spectrum 17 

Bell 26,  67 

Bitner 26,  27-29 

Birch-Hirschfeld 24,  67,  69,  70 

Bragg 4 

Bureau  of  Standards  Transmission 
Curves 43-51,  58-62 

Cady..  ..5 

Coblentz 5,  9,  17,  23,  26,  57,  61 

Continuous  Spectrum 13,  19 

Cornea,  Transmission  of 68,  69 

Crookes  Glass 25-26,  40-42,  56,  59 

Crystalline  Lens,  Transmission  of  .68-70 

Distribution  of  Light  Energy 7-9 

Various  Illuminants 17-23 

Edser : 14 

Effect  of  Thickness  on  Transmis- 
sion  52-55 

Emerson 26,  57,  61 

Energy,  Absorption  of,  by  Eye 75-82 

Density  in  Eye 80-82 

Fehr 26,  27-29 

Ferry  Quartz  Spectrometer 12,  30 

Fieuzal 61 

Fluorescence  of  Lens 71 

Fraunhofer  Lines .  .20-21 

Frequencies,  Table  of 3,  4 

Gage 37f  38,  39,  40 

Galvanometer 16 

Gibson 22,  26,  48,  49 

Glass,  Infra-red  Transmission 55-67 

Ultraviolet    and   Visible    Trans- 
mission   23-55 

Hallauer 7,  24,  61,  67,  70 

Ham 26,  27-29 

Handmann .  .  24 


Hartridge 72,  73,  75,  77 

Hartwell 5 

Heat 1-2 

Helmholtz 71 

Henker 24 

Herschel 5,  13 

Hertel 24 

Hilger 13 

Hill 72,  73,75,  77 

Hirshberg 55 

Howe 13 

Hyde 5 

Illuminants,  Spectra  of 17-23 

Incandescent  Solid,  Radiation  from. 8,  9 

Infra-red  Spectra 5,  13-27,  55-56 

Transmission  of  Glass 55-67 

Transmission  of  Ocular  Media.  .72-82 
Ives 5,  22 

Langley 6,  7,  9,  13 

Laue 4 

Light,  Color  and  Divisions 3 

Distribution  cf  Energy 7-9 

Wavelengths  of 2 

Luckiesh.  .7,  18,  19,  20,  22,  26,  33-37,  39, 

73,  75,  77-82 
Lyman 4 

McNicholas 22,  26,  48,  49 

Martin 67 

Media,  Transmission  of  Ocular — 67-82 

Meyhof  er 55 

Michelson 2 

Millikan • '. 4 

Nernst  Glower 17 

Nichols 6,  13 

Noviweld 59,  60 

Nutting 5,  23 

Parsons 67,  66 

Paschen 75 

Rund  Glass 49,50 

Radiant  Energy,  Glass  sources — 56-57 

Light  Sensation 7-9 

Methods  of  Producing  and  Inves- 
tigating  9-23 

Nature  and  Distribution 2 

Spectrum 4-7 

Roent  gen  Rays 4 

Rubens 6,  13 


84 


INDEX 


Schanz 24,  67,  69,  70 

Schulek 23 

Shaw 4 

Sheard 30,  32-33,  52,  62-67 

Smith 30,  32-33,  62-67 

Spectra,  Illuminants 17-23 

Representative 18 

Spectrographs,  Cornea 68 

Crystalline  Lens 68-69 

Infra-red 13-23 

Transmission  of  Glasses,  28,  29,  30, 
31,  32,  34-37.  39 

Ultraviolet 9-13 

Vitreous 70-71 

Spectrum,  Band 17 

Continuous 13, 19 

Infra-red 5 

Ultraviolet 5 

Visible 5 

Stearkle 23 

Steinmetz 6 

Stockhausen 24,  67,  69,  70,  71 

Temperature    Effects    on    Energy 

Distribution 7-9 

Thermopile 14-16 

Thickness  Effect  on  Transmission. 52-55 


Transmission,  Effect  of  Thickness . 52-55 

Infra-red 55-67 

Ocular  Media 67-82 

Ophthalmic  Glasses 23-67 

Ultraviolet  and  Visible , .  23-55 

Transmission  Curves,  Infra-  red,  58-59, 
61-66 

Ocular  Media 74-76 

Ultraviolet  and  Visible,  33,  37,  41-42, 
43-55 

Trowbridge 6 

Ultraviolet  Spectrum 5,  9-13,  23 

Ultraviolet  Transmission,  Glass .  .  23-55 
Ocular  Media 67-72 

Verhoeff 67 

Visible  Spectrum 5 

Transmission,  Glasses 23-55 

Ocular  Media. . 67-72 

Vitreous,  Transmission  of 69 

Vogt 24,  56 

Water  Equivalent  of  Eye 73 

Widmark 23 

Wood 6,  13 

X-Ray 4 


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